First, the ground rules. I cannot mention where exactly our meeting is taking place, so we agree on "somewhere in Ireland". Nor can I mention his current occupation, so we agree to refer to it as "his business", or similar. There is a third man in the room to act as a witness to our agreement.

It's understandable, I suppose, given that the man I am interviewing, Gilbert McNamee . . .

usually known as Danny . . . is the man who was accused of being the "master bomb-maker" behind the devastating 1982 Hyde Park blast which killed four members of the Household Cavalry. McNamee was found guilty in 1987 and sentenced to a 25-year jail term. He served 12 years before his release under the Good Friday Agreement. It wasn't until 1998 that his conviction for conspiracy to cause explosions was quashed on appeal and declared unsafe. "If people knew where I was, " he says, softly, "and what I do, well, it could cause some problems." Ground rules accepted. The third man leaves.

McNamee is almost unrecognisable from the widely used photograph of him as a 25year-old alleged IRA bomber . . . thick hair, narrow features and wispy moustache. A product of his republican background, he grew up in Crossmaglen, South Armagh, but he always denied being a member of the IRA.

Before his trial, in an unusual move, the IRA also denied that McNamee was one of their members.

Today, he is dressed soberly in a blue-grey suit, white shirt and red tie. Understated and opaque, he is balding, about 5ft 8ins and 45 years old, with a pointed determination in his eyes. He is as pale as a lily; a result, no doubt, of his years in a cell measuring two and a half by three metres, with little light. Sitting behind a large desk covered in papers and books, he looks a mix of withheld information and latent volatility. He talks a lot, but also listens. And he has a slightly unnerving habit of staring straight at you. If he stepped out of his office into a crowd he would be instantly invisible.

He could be anybody.

The bomb went off on July 20, 1982. As well as the four cavalrymen who died, 17 civilians were maimed and seven horses were killed.

The police began an intensive investigation, culminating in McNamee's arrest on August 16, 1986. Having graduated from Queen's University, Belfast in 1982, with a BSc in physics and electronic engineering, he was employed by Kimballs in Dundalk, making circuits for gaming machines.

During his trial, the Crown put forward in evidence three fingerprints and two pieces of electronic circuit board. The fingerprints were from a bomb left on a London street, and from sticky tape found in two separate arms caches, recovered in 1983 and early 1984. One of the circuit boards had been found in one of the arms caches; the other fragment was said to have been discovered after the bomb explosion at Hyde Park in 1982. The Crown's key scientific witness, Allen Feraday, said the two were matched in design and "artwork" and therefore made by the same master bombmaker. The prosecution based its case on the link between McNamee's fingerprint, the circuit board found in the arms cache, and the fragment of circuit board from Hyde Park.

When I mention Feraday to McNamee he looks directly at me. "I don't hate him, " he says. "I don't hate any of them. But I hate their methods."

McNamee's case was not the first conviction Feraday helped secure which was later overturned as unsafe. Feraday was severely criticised by the Lord Chief Justice in another case - Regina v Berry - before McNamee's conviction was finally quashed. John Berry, jailed in 1983 for selling timers to the Middle East, had his conviction quashed in 1993 after military experts challenged Feraday's evidence. The then Lord Chief Justice said the nature of Feraday's evidence in Berry's case was "dogmatic in the extreme" and "open to doubt at the very least".

In July this year, the conviction in a third case involving Feraday's expert advice was overturned. After a 20-year legal battle, the Lord Chief Justice ruled that the conviction of 53-year-old Hassan Assali, a Libyan, on terrorist conspiracy charges, was unsafe.

Assali's Hertfordshire factory was raided in 1984 and timing devices were seized. Feraday, the prosecution's only expert witness, said there was no lawful purpose for the devices, which Assali claimed were for domestic use.

"I am unable to contemplate their use in other than terrorist bombs, " Feraday told St Albans Crown Court at Assali's trial. After Assali's release, his legal team commissioned military experts from Berry's case, with backgrounds in explosives and electronics, whose subsequent report cast doubt on Feraday's evidence. The case was referred to the Court of Appeal in 2003, and the conviction was then overturned.

Perhaps more controversially, Feraday told judges in the case of Abdelbaset Ali Mohmed al Megrahi - the Libyan convicted of the 1988 bombing of Pan Am flight 103 over Lockerbie, in which 270 people died - that a fragment of a circuit board found in the wreckage was part of the bomb's detonator. The trial judges accepted his conclusion. In 2001, judges at a special court at Camp Zeist in Holland found Megrahi, now 53, guilty of murder. He was sentenced to life in jail. His co-accused, Al Amin Khalifa Fhimah, was cleared.

Feraday, now in his sixties, carried out some of the principal work on the key piece of forensic evidence at the Royal Armaments Research and Development Establishment (RARDE) at Fort Halstead in Kent. RARDE, the main UK forensic centre for examining terrorist incidents, was subsumed into the Defence Evaluation and Research Agency (DERA) after re-organisation. Feraday retired after 42 years' distinguished work.

Papers about Feraday's evidence in the previous cases have been sent to the Scottish Criminal Cases Review Commission (SCCRC), which is investigating Megrahi's conviction, and speculation is rife that the Libyan could be freed if the commission refers his case to the appeal courts. There are also reports that he might be repatriated to his home country. Last month it was reported that the British, American and Libyan governments were negotiating the transfer of Megrahi to a prison in his home country on condition that he drops his appeal, suggesting that both the British and American governments would rather the case was not re-opened. Tony Kelly, who represents Megrahi, refused to comment on the pattern of quashed convictions: "My client has taken the firm view that we will not comment on the case while it is with the SCCRC."

Having worked on both the Berry and Assali cases, forensic expert Major Owen Lewis (retired), who served with the Royal Signals for 22 years, is, according to one source, investigating crucial forensic elements of the Lockerbie case on behalf of Megrahi.

Throughout his career, Lewis, who had particular experience of the Middle East, acquired specialist knowledge of electronic warfare, triggers, improvised explosive devices and surveillance.

The source said: "By now he's already got the modus operandi. And he knows how it works. Lewis is a very shrewd man, a very clever man." Kelly, Megrahi's lawyer, steadfastly refuses to comment.

'The first thing I would say is that I don't know the effect it has had on me, " says McNamee, with beguiling calm. "But I'm very uncomfortable with being portrayed as a victim who has been irreparably damaged.

You do, of course, suffer, and different people experience prison in different ways." He pauses briefly. "I have no doubt from a social interaction and a psychological point of view it has affected me, but I'm the wrong person to give an analysis of how. I'm happy being busy and I don't actually think about it all the time.

I prefer to think of it as an experience in my life and I survived. And I go on."

It would be hard to overstate the impact his sentence had on his personal life, however.

McNamee was due to be married a week after his arrest, but the relationship fell apart after he was imprisoned. "I had a child who was born just shortly after I went to prison and who I didn't see for a great deal of time." He looks away before tentatively revealing a few bits of information he'd rather keep to himself. "You are constantly aware you don't know your child as much as you should. I think it had a very negative impact on her life. She was more angry with me from the point of view that she doesn't look at the reason why I was in prison, she just knows that I was not there [with her]." He has subsequently married someone else. "But I'm separated."

He won't go into any more details.

The way he looks at his world now, he says, is the way he looks at life in general. "You can always identify people who are worse off than you. If I didn't do it this way, I'd be falling into the self-pity phase." Did it ever come tumbling down? "Not yet." He looks at me. "Not yet."

He talks about his case, both on and off the record, and about those involved in his conviction. "The whole thing with forensics is that it poses the question: does it follow what the police want it to say? Or is it an independent thing which does give up independent results?

In this particular case they [the police] were wanting the evidence to the effect that it would get a conviction. The net outcome is that evidence is unreliable."

McNamee appealed on eight grounds, one of which challenged the reliability of the evidence given by Feraday, the prosecution's principal forensic witness at his trial. The case was referred back to the Court of Appeal in 1997 by the Criminal Cases Review Commission after inquiries into a number of issues, including disclosure of evidence at the time of his trial, scientific and fingerprint evidence.

Dr Michael Scott, now a senior lecturer in computing at Dublin City University, gave evidence at the High Court in London where McNamee appealed his sentence. Scott has a degree in electronics from Queen's University, Belfast, and in engineering from Trinity College, Dublin. In 1977 he finished his doctoral dissertation at the University of Dundee. In spite of heading the government's explosives unit, Feraday's only relevant qualification was a Higher National Certificate in Applied Physics and Electronics. Throughout his career, however, he has spent a number of years studying explosives, and also specialised in analysing the capability of the IRA. He has also given testimony in many cases where his evidence was upheld. In June 1989 Feraday was made an OBE in the Queen's birthday honours.

When I contacted Scott in Dublin, he told me: "Taking circuit boards out of the explosives context, which in many cases was appropriate, then any number of electronic engineers would be better qualified than Feraday. Feraday's most damning conclusion was to point at a piece of electronics and say that it was part of a bomb, a purpose for which it was specifically designed and constructed, and that it could not be for any other purpose.

"However, his knowledge of electronics is in fact elementary, and his conclusions often just plain wrong. The electronics indeed could have other uses. His advantage is his explosives experience. However, even in this context there would be others better qualified than him. At the Berry appeal, where Berry had access to British army expertise, Feraday's evidence was, if you will excuse the expression, completely blown away."

Scott also described as "just nonsense" Feraday's assertion that the circuit board found in McNamee's case could only be used for bomb-making. "The simple circuit board found in this particular context could have had many other uses. Indeed, it was just an amplifier board, which is itself just a component. Just because an alarm clock can be used to make a bomb, it doesn't make possession of an alarm clock tantamount to possession of a bomb."

Feraday and Scott had crossed swords before. In September 1988, at an inquest into the deaths of three IRA members killed by the SAS in Gibraltar, Feraday claimed a device soldiers mistakenly thought was in the terrorists' car could have been triggered from anywhere in Gibraltar, or even from Spain.

The security forces believed Feraday's evidence would be crucial in establishing why the SAS believed the IRA members could have detonated a bomb after the SAS closed in. The shooting of the three terrorists was justified under law, an inquest jury decided in October 1988. However, in September 1995 the European Court of Human Rights judges ruled that the shooting of the three IRA men by members of the SAS in Gibraltar involved the use of excessive force.

The voice at the other end of the telephone is more upbeat than I expected. "I'm just getting on with my life, " says Hassan Assali, buoyantly. It's just over three months since his conviction on terrorist conspiracy charges was ruled unsafe, following a 20-year battle to clear his name. Having lost his house, his successful business and his first wife (he was divorced while in prison but has since remarried), he now lives in rented accommodation in Surrey. He is reluctant to discuss his case now that he intends suing the Crown for his false conviction, but he believes his freedom has given him his own sense of moral justice.

"I lost everything, " he says. "I was a person of good character who never had anything more than a parking fine to my name. But I knew I was right. My daughter, who was about five when I was jailed, knew I'd come out on top. When [the verdict] got quashed she was in tears and said, 'I knew you were going to do it.' Well, I had to fight and fight and fight. But I never felt anger. Being an Arab it was par for the course."

Assali was born in Libya in 1951. In 1966 his family moved to the UK, where he completed a four-year degree course in electronics. Later he gained a master's degree in mechanical engineering. In 1978 he opened his own company, Radiofort Sentek Ltd, based in Borehamwood, Hertfordshire. The company manufactured various electronic devices, including sophisticated security and tracker systems; its many clients included the Ministry of Defence.

In May 1984, police and customs officers searched Assali's home and factory following a tip-off by a disgruntled employee. The searches took place in the highly charged atmosphere following the shooting of WPC Yvonne Fletcher outside the Libyan People's Bureau the previous month. "Feelings were running high because of the Fletcher case, " says Assali.

"They [the police] did war-dances around me for months. It was a load of bollocks."

Seventeen timers were reportedly found in an airing cupboard in his home, and seven half-assembled timers at the factory. At his trial, Assali stated that these timers were not designed to be used in conjunction with explosives. The prosecution's case was that he had been using his company to produce timers which were explosive devices. On May 24, 1985, he was convicted of making explosives, contrary to section four of the Explosive Substances Act 1883, and sentenced to nine years' imprisonment. "There was nothing, " says Assali. "There was no fire to generate any smoke. They just vented a few dirty carpets and made some smoke."

He served six and a half years. After his release, his legal team commissioned military experts . . . Major Lewis, Lt Colonel John Wyatt (retired; a 23-year veteran of the Royal Engineers, involved in bomb disposal and counter terrorist operations) and Squadron Leader Michael Hoyes (retired; a chartered engineer who spent 22 years with the RAF) . . . whose report cast doubt on Feraday's evidence. As a result his conviction was overturned by the Lord Chief Justice.

According to the Appeal Court judgment:

"There is no doubt that an important part of the Crown's case against the appellant [Assali] depended on the evidence of Mr Feraday . . .

He examined all the devices that had been recovered. His evidence supported the Crown's case with regard to the nature of those devices."

The judgement also cited the Berry case, which had similarities to Assali's. It stated:

"On the appeal in that case, evidence was given by Major Lewis and Colonel Wyatt, together with Dr Bora, who were highly experienced and impressive court experts who concluded that similar devices to those in this case were simply timers. Mr Feraday had also given evidence in the case of Berry. The evidence which was given by the three experts to whom we have just referred rebutted the evidence of Mr Feraday that the absence of safety devices in the timers prevented their use for legitimate purposes.

Accordingly, the Court of Appeal concluded in Berry that Mr Feraday's opinions were central to the trial and were open to doubt at the very least. They therefore quashed Mr Berry's conviction. As the evidence of Mr Feraday was equally crucial to the prosecution in this case, the implications for this case were obvious."

Of Allen Feraday, Hassan Assali simply says: "He's a very, very experienced evidencegiver. If his evidence managed to convince a judge, he must have been bloody good."

Part of the prosecution's response to the Assali appeal stated: "Critical to the case against the appellant [Assali] was Allen Feraday's evidence. The Crown is of the view that there is a reasonable argument to suggest that the . . .

material [meaning the report by Assali's defence experts] might well have left his [Feraday's] evidence open to reasonable doubt. In the circumstances, the Crown does not feel it is in a position to advance argument to support the safety of the conviction on this basis, and will not seek to resist the argument of the appellant that this material renders his conviction unsafe."

Assali was officially a free man on July 19, 2005.

Assali believes the successful challenge to the evidence in his own case "will have significance on the Megrahi case". He also believes his own case was delayed in a bid to prevent Feraday's evidence being scrutinised before Megrahi's appeal. "The authorities didn't want to rock the Lockerbie boat. This bollocks about Megrahi . . . absolute shit. I know there is some devastating stuff. The SCCRC will have it at the moment. And they INVESTIGATION can dig up further because they have extreme powers.When that comes up . . . by God. That's the satisfaction I have at the moment."

Earlier this year, a senior Scottish police officer, now retired, was reported to have accused US agents in the Lockerbie case of planting a fragment of circuit board. The officer is believed to have given a signed statement to Megrahi's lawyers. The circuit board fragment was confirmed by Feraday as part of the detonator of the Pan Am 103 bomb.

The cases of Assali and John Berry, a former Royal Marine Commando, were similar. In 1983 Berry was sentenced to eight years, later reduced to six, after his company JJ Security allegedly supplied 1,000 electronic time-delay systems to Syria. He was convicted of making explosive devices for terrorist use in the Middle East and spent three and a half years in jail.

Allen Feraday, the Crown's expert witness at the trial, declared of the timers: "I am of the opinion that they have most probably been specifically designed and constructed for terrorist purposes. I am unable to contemplate their use other than in a bombing context."

In September 1993, Berry won his battle to clear his name when his conviction was quashed by the Court of Appeal. The court ruled his conviction was unsafe and unsatisfactory because fresh evidence cast doubt on unchallenged scientific testimony that the electronic timers could have only a military use.

Berry always maintained that the timers had been supplied to the Syrian government and that they had numerous applications. The Appeal Court held that the devices were supplied under an entirely legal contract, and that the trial judge of 1983 had been wrong to maintain that they were going to be used in terrorist attacks. The Syrian government had, moreover, returned the devices . . . on the grounds that they were defective.

The appeal judge said Feraday had reached the "extremely dogmatic" conclusion that the timers could only have been designed and made for terrorist bomb use. Four experts in the appeal contradicted that evidence, saying the timers could have had a variety of uses, including surveillance, and were not made specifically for "terrorist use".

Feraday's conclusions were described as "uncompromising and incriminating". In 1993, following the quashing of his conviction, Berry said: "I feel I have been treated so badly by the people who operate the system." The conviction led to his divorce. The Home Office later agreed to pay him compensation from the public purse.

Berry, now 70, is in Spain, where he lives for part of the year, and could not be contacted.

According to a friend, he has put the past behind him. "He's a successful man, just getting on with his life. He plays a lot of golf."

Victoria Cummock's husband John died on Pan Am flight 103, leaving her widowed with three small children. Her American lawyer, Jodi Flowers, doesn't believe the latest revelations will have too much impact on the Lockerbie case. "Simply stated, " she says, "Megrahi was not convicted merely on the testimony of Mr Feraday. Mr Feraday has been involved in many, many cases."

Allen Feraday lives in Halling, in Rochester, Kent. He was invited to comment when contacted at his home, but declined.

Danny McNamee lives a life now where people don't really know about his past.

"Because of my background, " he says, "you still have the baggage. People are not looking at things in an objective way at all; they are looking at them in a tribal way. That's the problem I've got in relation to not being more open." His health, more than anything, has been affected. He developed psoriasis in prison, and still gets chest infections following a serious illness contracted in jail. His compensation claim is ongoing. "The Home Office hope you die before you get it, " he says.

"The system cannot handle someone who says, 'But I didn't do this, boys.'" Curiously, he says he always knew the legal process . . . though not necessarily British justice . . . would free him. "Not in those terms, " he explains. "What it did do was make me understand the value of the legal system being properly implemented. The rules are there to be followed. If there was anger, it was towards the people who should know better . . . the people who don't obey the rules and they know the rules."

Before finishing up, he talks briefly about the Lockerbie case and the impact his own case . . .

and the cases of Berry and Assali . . . might have on it. "They know Feraday's judgement is, at the very least, questionable, " he says, his voice weighed down by his past. "But you have to ask not really a question about him, but a question about the prosecuting authorities who then seek to rely upon someone whose evidence has been discredited."

He shrugs. His life has moved on. It's not come tumbling down. But the ripples of his story are still being felt. Abdelbaset Ali Mohmed alMegrahi is serving his sentence in Greenock Prison near Glasgow, where he continues to protest his innocence. The case is being considered by the Scottish Criminal Cases Review Commission, whose findings are expected to be announced early in 2006.

According to one source, "the unmasking of the judicial system and all its hubris will be there for all to see".