Through all the gritty subject matter – an undercover investigation into allegations of police racism, exposing the murky financial affairs of Rangers and taking blood-boosting drug EPO to examine doping in sport – Mark Daly insists it is a lone mantra that drives him: to make a difference.

There is a fleeting blush, an almost apologetic smile. The BBC Scotland investigations correspondent is all too aware that it could sound trite and twee.

Yet, it is something which has motivated Daly, 41, since his early days as a trainee reporter in the 1990s when he conducted a newspaper campaign to highlight the plight of victims of asbestos-related conditions in his home town of Clydebank.

By his own admission, Daly is a reluctant interview subject. It is the proverbial case of poacher-turned-gamekeeper (or should that be the other way around?) as we settle into a quiet booth of a Glasgow coffee shop to shine the spotlight on his life and career to date.

There are times when he is physically cringing at the process, but valiantly perseveres. "I am in your hands," he says, sounding about as relaxed as someone in the dentist's chair about to undergo an afternoon of root canal treatment.

Daly was thrust into the public eye in 2003 after being arrested while conducting an undercover investigation for the BBC into allegations of racism in the police service. Things weren't quite meant to pan out that way.

Prematurely rumbled by internal affairs, Daly found himself making headlines for all the wrong reasons. It was a period which he says saw "an incredibly dark PR campaign" waged against him and the BBC.

That all changed when The Secret Policeman was broadcast in autumn 2003. The film sparked a political row which led to 10 officers resigning and 12 more facing disciplinary action.

Daly had immersed himself in that world for almost two years, stripping away large chunks of his own identity in order to go "deep cover". He admits that no matter how many books read on the subject, psychological tests conducted or role play sessions performed, it was impossible to be prepared in advance for what unfolded.

He went undercover as a trainee officer and secretly filmed at the Bruche National Training Centre, in Warrington, Cheshire, and also on patrol in Greater Manchester. "I had all this James Bond kit with cameras everywhere including in my bulletproof vest, briefcase, dashboard of the car, stereo and whenever possible had a covert crew following me on the beat," he recalls.

"It was literally like living a double life. I was in constant fear of making a mistake. There were a lot of people counting on me. I was worried I was going to make a mess of it. That I would leave a wire hanging out or fail in some way."

Daly was aware the "good cops" he had trained alongside – those who weren't racist – would feel betrayed. That niggling guilt made the subsequent emotional fall-out less cut and dried.

"The difficulty with long-term undercover is that, unless you are made of steel and a robot, you are going to make connections with people," he says. "You spent 24/7 living on top of guys and bonding in this unrealistic environment. It was one part army training camp and one part Butlins."

Then came an invitation to speak at the National Black Police Association annual conference in 2004 where Daly received a standing ovation from 600 officers. "They told me I had changed their lives and had been waiting for that for decades," he says. "That helped me deal with the fall-out of betraying people I had become close to. Ultimately, I felt that what I was doing was worth it."

Daly has gone on to work on high-profile BBC investigations including Rough Justice: Murder Without A Trace about a miscarriage of justice involving Barri White and Keith Hyatt who were wrongly convicted of murdering 19-year-old Rachel Manning from Milton Keynes in 2000.

The programme, which aired in 2005, played a pivotal role in White and Hyatt's convictions being quashed in 2007. Six years later, Shahidul Ahmed was convicted and sentenced to life in prison for killing Ms Manning. Daly describes it as "the most important journalism I have ever done".

Other headline-making projects include the Scottish Bafta-winning documentary Rangers: The Men Who Sold The Jerseys, which revealed the details of payments made to players and staff of the football club through offshore Employee Benefit Trusts (EBTs) between 2001 and 2010.

More recently Daly fronted Panorama specials Catch Me If You Can, which delved into allegations of doping in sport spanning more than 30 years, and Seb Coe and the Corruption Scandal, in which he investigated how much Lord Coe knew about corruption and doping cover-ups in athletics.

Daly loves sport but concedes his recent forays have taken the gloss off watching it as a fan. It is a subject matter, however, that he finds too compelling to step back from.

"There is a never-ending trail of great tales to investigate," he says. "For so long, the people in the corridors of power in sport believed it was their domain to run it as they saw fit. They don't like scrutiny – they hate it.

"I believe sports administrations are on their toes now. WADA (World Anti-Doping Agency) is beleaguered, the IOC (International Olympic Committee) is under the most pressure it has perhaps ever has been, Fifa and the IAAF (International Association of Athletic Federations) have their own issues. It is very fertile ground and I'm struggling to pull myself away from it."

He refuses to be drawn on the issue of the contentious Scottish Six news programme other than to acknowledge the potential to "bring dozens of new jobs at a time when the industry is on its knees".

The youngest of five children, Daly comes from Clydebank. His father was a shipyard worker and late mother a housewife. With an older brother who required extra care due to a disability, Daly recalls being acutely conscious throughout his life of not troubling his parents with trivial matters.

He was a naturally inquisitive child always in search of adventure. Daly laughs when asked about embarking on Famous Five-style escapades. "I grew up in Clydebank – it wasn't quite five go off on a jolly adventure with lemonade and a wee treasure map," he grins. "I went to what could be described as quite a tough school. It was hard. I wasn't in the cool clique. I was a bit of an outsider."

While the men in his family had traditionally gone into the shipbuilding trade, Daly studied film and media at the Stirling University. It was there his love of journalism blossomed. He went on to complete a postgraduate diploma in journalism at Strathclyde University and got his first byline in a national newspaper writing about swimming for The Herald in 1998.

Daly landed a job on his local paper The Clydebank Post and worked there for a year before being moving to The Scotsman and then the Daily Record. He joined the BBC in 2002 and his prolific investigative journalism career took off.

Daly is gregarious company although fiercely guarded of his personal life. Hardly a revelation given the torrent of online abuse he has faced over the years as a result of his investigative work. He doesn't dwell on that aspect, seeing it as part and parcel of the job. In 2012, a man was charged and later fined after making threatening remarks to Daly on Twitter.

He is married to Seonaid, 35, who works in the arts ("probably my biggest fan and fiercest critic") and the couple have two children.

Daly was a keen swimmer throughout his childhood and swam competitively at university (in the same hallowed waters at Stirling now graced by global stars such as Ross Murdoch and Robbie Renwick no less) where he was known to post a sub-one minute in the 100m freestyle.

In more recent years, Daly has completed two Ironman races and enjoys cycling although family commitments have seen his bike forlornly gathering dust of late.

Daly says he rarely fully switches off from work and is a self-dubbed perfectionist. "Would people call me a control freak?" he ponders. "I'm very much across all the elements of the production. I like to be in the edit, write the scripts and do the meetings with the lawyers …"

Nor could he imagine doing any other job. Given his insatiable appetite for chasing stories (fuelled by an in-built trait of being "incurably nosy"), Daly typically has five for six projects on the go at a time. "It is addictive," he says. "If I was to end my career tomorrow I would be happy with what I have achieved. But I can't think of anything else that would give me the same amount of satisfaction."

LIFE AND LOVES

Career high: Rough Justice documentary resulting in Barri White, wrongly convicted of murdering his girlfriend, being freed.

Career low: Independent Police Complaints Commission (IPCC) finding against our film over Met police corruption in the Stephen Lawrence case.

Favourite film: All The President's Men and Jaws.

Last book read: The Laidlaw Trilogy by William McIlvanney.

Best trait: listening.

Worst trait: procrastination and being late.

Best advice received: If you're talking, you're not listening.

Biggest influence: Graham Crawford (first editor at Clydebank Post), Jim Wilson (editor at The Scotsman), Simon Ford (first boss at BBC) and Murdoch Rodgers (producer for the past five years).

Favourite meal: Steak frites.

Favourite holiday destination: France.

Favourite music: Techno.

Ideal dinner guests: Ernest Shackleton, Doreen Lawrence, Ben Bradlee, Joe Pistone, William McIlvanney, Andy Murray and my wife.