If someone gets their final salary from an employer after they have left that employer, what (if anything) can the company do to get that money back?

Does the ex-employee have a legal responsibility/duty to alert their ex-employer of the overpayment? Can their ex-employer claim ‘theft’ if they don’t? Can they claim interest on the money if they realise their mistake at a later date & ask for it back?

Or can the ex-employee just keep the money?
Fred Roberts