Do you have to repay?
Generally yes, even if it is the employer/agency who has made the mistake, the money will need to be repaid. The statutory protection against unlawful deductions from your wages, contained in the Employment Rights Act 1996 and the Employment Rights (Northern Ireland) Order 1996, does not apply where your employer has made an overpayment of wages.
If you are leaving your job and have been told you owe your employer money for previous training, see our repayment clauses section in Contracts.
You should only be required to repay the amount of overpayment that you actually received. It is down to your employer to make arrangements for the recovery of tax and National Insurance.