Oh dear! I don't envy you.
Lets just hope you're not 1p over because unless you're a member of the cabinet in your spare time (I'm thinking about Blunkett's travel fraud affair), you're going down!
I wonder if you claimed that you had misplaced some receipts you would just have to re-pay the amount concerned or would they actually send you via the disciplinary route. I would imagine that it would be pretty hard to prove fraud in such a case? Maybe someone can enlighten me?
Good luck!