Coulson: 'Perjury has to matter to the case'

Secondly, why did the judge decide to dismiss the case - ie without determining whether Mr Coulson had lied during Tommy Sheridan's trial?

The starting point for the first question is that the Crown has to prove its case. It needs to ensure that enough relevant and admissible evidence goes before the court to allow the jury to make a decision.

The decision on sufficiency and relevancy of evidence is a legal question which is the judge's - and not the jury's - job to determine. The same principle applies in non-jury cases and is one of the fundamental principles upon which our criminal justice system operates.

The decision of a judge or sheriff to hold that there is no case to answer is often based on a lack of corroboration or if the evidence used is held to have been recovered illegally - for example by coercion or on an illegal warrant.

In this particular case, turning to the second question, it seems that the judge's decision was based on the irrelevancy of the evidence from the original Sheridan trial.

The Crown has to prove that the evidence which Mr Coulson gave was relevant to that case. The defence in this case satisfied the court that phone hacking was not a material consideration.

Apparently, Mr Sheridan had himself conceded as much in his closing speech to the jury, arguing that he had brought Mr Coulson to court "to expose wrongdoing", an entirely separate matter from whether or not he himself had lied in his defamation case. This proved to be a fairly compelling argument.

Perjury, therefore, has to matter to the case. I am not convinced that the decision has in any way helped Mr Coulson's reputation, but it does help us to understand more fully the extent to which the Crown must go to prove a criminal charge and the hurdles it must cross before the jury can even have its say.

David McKie is a partner with Levy and McRae solicitors in Glasgow and is Head of Media Law at the University of Glasgow.