Overpayment of wages
A. Yes, in most cases like this you will have to be prepared to pay the money back.
The Employment Rights Act 1999 has a section that says that it is not unlawful for an employer to deduct overpaid wages or expenses, made (for any reason) by the employer to the worker (including a mistake made by your employer).
If you are worried about whether you can afford the repayment immediately, you should speak to your union rep to help you discuss repayment with your employer.