You know, I don't really think that's a great idea. Like it or not, the British MPs are doing a job, and like every job I know of, there are expense accounts for the upper management. As long as their expenditures are judged necessary and non-extravagant by the accounting department, they don't need to pay them back.
The issue is really corruption and incompetence. For the first portion of blame I push that onto the people. The restrictions placed on Parliamentary spending is up to the people to dictate and oversee. If an elected official is violating this trust, it is up to the people to hold them accountable. The second part also falls on the people. If someone proves to be unable to lead from the position which they have been appointed, it is the people who should terminate that persons employment.
No, I do not think that they are the ones who need to pay the people back. I think that he government should be the ones that have to pay, since they did not do a whole lot wrong, they just had some small things that went wrong with them.
As an American I feel it is odd to suggest that all prime ministers should pay back their parliamentary expenses that they incurred while in office. That's simply something we don't do here as those expenses tend to be fully covered for politicians. I think it's an interesting suggestion to save money, but what happens if they can't pay it?
It would not be right for British MP's to have to pay back expenses that they spent while doing their jobs as part of parliamentary work. If some of those expenses though are proven to be callous and wasteful, then I think the government should decide if a former British MP should be fined.