I posted this some time ago on a different thread. The exact circumstances may be different in your case but it does work if your case meets the criteria.
A note about the RAF claiming money back that was incorrectly paid. There is a legal defence of estoppel which could (in the correct circumstances) apply. This is based on mistakes of fact by the paying party (the RAF):
Firstly, if it is established that the overpayment resulted from a mistake of fact, there may be an obligation to repay some or the entire amount overpaid.
However, To argue successfully against repayment, the payee must be able to show the following:
• They did not contribute to the error, for example by providing incorrect information to the employer. This may scupper you claiming for something that you could have known by reading the JSP was not yours to claim.
• They believed in good faith that the money was really theirs. This will be easier to argue successfully where a relatively small amount was paid over a period of several months. An additional £5.53 paid every month for two years could more readily be argued to meet this criterion than, for example a one off bonus of £2,000, which the payee could reasonably be expected to notice and query.
• That the payee, believing the money to be rightfully theirs, has “changed their position” i.e. spent it. It should be possible to show that the money has been spent on day to day expenses and incorporated into the payee's general budget.
If the payee meets these criteria it can be argued that the employer is “estopped” from deducting the amount overpaid from the payee's salary.
Estoppel is recognised in government accounting and this worked for a SAC who whilst working for me had been overpaid due to an admin cock-up, when they tried to claim it back the Chf Clk helped him draft the letter and the 800 odd quid was written off.
There is a legal precedent in the case of Avon County Council v Howlett (1983).
I hope this helps, and as I say, I know it works - I have seen it (and have a copy of the letter sent to the SAC confirming it).
Good luck.