The employer in this case fell into the trap of assuming that, as long as it waited for a while (one year in this case) after a TUPE transfer, it could detrimentally alter the contractual benefits of employees who had transferred, in a bid to harmonise its workforce's terms and conditions.
A model letter to explain to a former employee that a deduction is being made from a final salary payment to take account of an accidental overpayment discovered after they left your organisation.