The principle is simple enough: Parliament doesn't interfere with the courts and the courts don't interfere with Parliament. But it is not unlimited: a member who stole money from another member's room could expect to be tried in the ordinary courts. The question for Mr Justice Saunders was how far privilege extended to matters that were ancillary to the main work of Parliament. 

Not very far, was his answer. Lord Pannick QC, for the prosecution, conceded that the expenses scheme itself was covered by privilege. So, perhaps, was the administration of that scheme by officials. But privilege did not cover the submission of a claim form by an MP or peer. "The fact that it is the submission of the claim form that sets the machinery of Parliament in motion does not make it part of that machinery just as putting a coin in a slot machine does not make the coin part of the mechanism of the slot machine," Mr Justice Saunders said.

On this point, the judge's ruling was pragmatic rather than principled ("there has to be a line drawn and it has to be drawn somewhere"). His response to the defendants' second main argument was equally pragmatic, though more convincing.

That argument was "that in order to exercise freedom of speech in Parliament, it is necessary to be able to attend; and therefore expenses are ancillary to the exercise of freedom of speech and are included within the definition of proceedings in Parliament within article 9".

Following recent legislation, freedom of speech in Parliament extends to the preparation of documents "incidental to" business before Parliament. But the judge said it would be stretching the meaning of "incidental" too far for it to cover a claim for expenses.

Let's see if the appeal judges agree.