So the MPs facing prosecution (see this excellent blog for more detail - Law and Lawyers) want to rely on parliamentary privilege.
This is very clever because nobody actually has a clue about how far parliamentary privilege goes. Don't try looking in Erskine May because the relevant section provides more questions than answers. In the recent case of an MP's room in the Commons being searched it became apparent that lawyers found it difficult to say whether or not there had been a breach of privilege (see the memo of the Clerk of the House). I suspect that the expenses prosecutions will all centre around the definition of ' proceeding' in article IX of the Bill of Rights - this was the Privy Council trying to work it out in 1963 ( A.C. 103 Page 121):