Any MP who cannot explain his or her expense claims to their constituents should pay that money back - end of chat.
The following is from the Green Book (Parliamentary Salaries, Allowances and Pensions booklet):
Use of Parliamentary allowancesMany MPs have ripped the taxpayer off by working "within the rules". Those MPs should shut the hell up and think themselves very lucky to still be in our employment.
It is your responsibility to satisfy yourself when you submit a claim, or authorise payments from your staffing allowance, that any expenditure claimed from the allowances has been wholly, exclusively and necessarily incurred for the purpose of performing your Parliamentary duties. The Parliamentary Commissioner for Standards may investigate allegations of improper use of the allowances.
If you overspend, or mischarge something to your allowances, we reserve the right to deduct the costs from your salary or, if you are leaving the House, your resettlement grant.
Director of Finance & Administration