Expenses: MPs cry foul at payback demands
"Their argument is that these claims were within the rules when they made them and that to force them to pay back money now is retrospective justice"
I think you'll find they argue that it is retrospective Injustice.
Let's imagine you work for a company who says your expenses limit for an hotel is 100 pounds per night. Four years later, they say: "Look, we've changed our mind. We now think the limit should have been 80 pounds, so you owe us 5 grand." How would you react?