PAYE76135 | PAYE operation: pensioners: recovery of overpaid pensions
From HM Revenue & Customs · PAYE Manual
You will not normally be told by the Department for Work and Pensions (DWP) if a pension is revised.
Follow the instructions in the Employment Income Manual if a pensioner disputes the amount of pension being taxed because
He / she has been asked to refund some of the pension already received
Or
The current pension is reduced because of overpaid pension in earlier years
Where the current pension is reduced, you must assess or code the full amount to which the pensioner is entitled.