Who, then, is to decide whether parliamentary privilege provides a defence for the three MPs? First, the privilege belongs to Parliament — and not to an individual MP. So Parliament can waive it in an individual case. 

But even if Parliament were to conclude that claims for expenses were proceedings in Parliament within the meaning of article 9, that would not, by itself, have any effect on their admissibility in a criminal trial. This was the view of the Attorney General, Baroness Scotland, less than a year ago.

"The material will only be inadmissible if the courts consider the use to which it is put amounts to the ‘impeaching or questioning' of Parliamentary proceedings," she explained in a memorandum to Parliament dated 3 April 2009. "It would be unprecedented for the House itself to resolve that the material is being put to such a use — the House would not know the use to which the relevant material is intended to be put without questioning the prosecuting authorities."

In any event, she continued, it was for the courts to decide whether article 9 rendered evidence inadmissible. "Article 9 is statute law and its interpretation, as with any other statute, is a matter for the courts."

It's clear from this that the courts can assess the evidence in these cases — MPs' claim forms, presumably — and conclude that treating it as admissible would not impeach or question the expenses system that was then in force. 

On 8 February, the Conservative leader, David Cameron announced that he had asked Sir George Young, the Shadow Leader of the House, to prepare the legislation that the Nicholls committee had recommended in 1999. A Bill would be introduced as soon as possible "to clarify the rules of parliamentary privilege to make sure that they cannot be used by MPs to evade justice".

Clarification would obviously be desirable. But the legislation could not affect the three MPs and peer in question, Elliot Morley, David Chaytor, Jim Devine or Lord Hanningfield. If what they are alleged to have done was lawful at the time, criminalising it retrospectively would be both unfair and a breach of the Human Rights Convention.

Fortunately, parliamentary privilege seems to be of no use to these defendants. They should do the decent thing and say that they will not be relying on it in court.