Overpayment of wages
A. While generally your employer cannot deduct money from your wages without written authorisation — see the answer to the question above — there is an exception in the case of overpayment of wages or expenses. Section 14(1) of the Employment Rights Act specifies that the unauthorised deduction provisions do not apply in cases of overpayment.
If you’re satisfied that you were overpaid, you can argue that the deduction should be made over several months — this is especially since the overpayment relates to an event so far in the past — if it would put you into financial difficulty to have to make the repayment in one go.