Is Andy Coulson a liar?

The former tabloid editor denies giving false testimony at the 2010 trial of ex-MSP Tommy Sheridan.

Mr Coulson, who edited the News of the World, denies that he lied on oath when he said that he first know of phone-hacking at his newspaper when his royal editor was arrested in 2006. This, despite his 2014 conviction conspiracy to intercept voicemails during his time at the now defunct title.

The 47-year-old has now been cleared of perjury. But whether or not one-time Downing Street spin doctor told the truth at the trial of Tommy Sheridan back in 2010 has not been put to a jury. Nor will it ever be.

That is because Lord Burns, at the High Court in Edinburgh, ruled that Mr Coulson had no case to answer.

The judge's logic: that allegedly false testimony was not relevant - that is, legally material - to the original case against Sheridan.

The former politician, representing himself, had called Mr Coulson as a witness for his defence and peppered him with a whole range of questions about phone-hacking and other alleged wrongdoing at the News of the World.

Was this strictly relevant to his case? Well, Sheridan thought so. Four years earlier he had successfully sued the News of the World for £200,000 over stories reporting that he had gone to swingers' clubs and had affairs.

The Crown thought he had lied about this. It had a tape - made by Sheridan's former friend George McNeillage and purchased by the News of the World - that appeared to show Sheridan admitting the stories.

Sheridan thought the culture and conduct of the newspaper was vital to his defence against perjury charges: that he was the victim of a conspiracy.

The Crown, of course, at the time didn't think this was relevant.

In an ironic turn of events, it does now. Well, as people in Mr Coulson's line of work used to say, up to a point.

A senior Crown source, speaking to The Herald on condition of anonymity, explained the prosecution thinking.

The testimony was admitted, he said, without objection from the Crown or any intervention from the then trial judge.

The source added: "Sheridan's defence was that he was the victim of a News of the World conspiracy which involved Coulson ordering Sheridan's phone to be hacked and that the McNeilage tape was concocted.

"As phone hacking of Sheridan's phone was part of the conspiracy then any evidence that Coulson, as editor, knew that phone hacking was going on in NOTW was material and relevant evidence.

"So Coulson's denial that he knew about phone hacking was material and relevant evidence. The fact that this evidence was false - allegedly - provided the basis for the prosecution.

"The trial judge is of course entitled to form his own view of relevance, which he did."

What exactly does it mean for evidence to be relevant? Some believe it means the testimony has to have actual effect on the outcome of a case. But that theory was overturned 30 years ago. No, relevance of evidence is just what it sounds like: testimony that has a bearing on the facts of the case, whatever the outcome.

Relevance is a matter for a judge, not a jury. So could the issue have been heard earlier in the court process, saving the expense of a trial? That was mooted, said the Crown source. "It was continued to the trial diet to hear the evidence before deciding the issue."

Legal experts, meanwhile, are relaxed about the Scots Law definition of perjury. Why? James Chalmers of Glasgow University explained: "You might think that perjury should be any lie in court. But, in general terms, if somebody lies on an irrelevant detail, it is probably not in the public interest to prosecute them."