My understanding is that, if the overpayment was the result of a genuine mistake on their part (i.e. it wasn't paid as an extra and they have simply changed their minds), they are entitled to recover the money. However, they must give you the exact amount and show how they arrive at this figure. If you agree the figure, then you should offer repayments at a rate you can afford. If they do not accept this, their only option would be to sue to recover the money, at which point you would make the same offer, which would probably be accepted by the court and the employer would be left with court costs as there was no need to bring the case to achieve this result, IYSWIM!!