Facing wage deductions after overpayment

Q: Over the last 4 months, my employer has overpaid me and I have been paid an extra £300. I will now potentially face a deduction from my pay to bring it back into balance, but would like the sums to be fairly small. What are my rights here?

A: Payments made under a mistake of fact or law are generally recoverable, especially if there is a provision within the contract. However, if you in good faith have changed your financial position by incurring expenditure which you would not have otherwise incurred, you may be able to reject this deduction. The difficulty would be the evidential burden of proving this. The request to deduct your pay in a smaller amount is certainly a reasonable request in light of their mistake.