Your mother's pension entitlement will have ended at her death. If the DWP overpaid due to an administrative error, they will probably say you should have noticed. Does the Post Office Account statement itemise this payment in detail?
You should take this whole matter and any paperwork to a Citizens Advice Bureau/other advice agency and ask them if you have to pay the money back. Either way they can help you deal with the DWP debt collectors. You should do this asap as interest charges on the amount involved will probably be added. Best wishes.
Thank you for reply - I called them yesterday and they said that if money had been used in Estate and the Estate had not gone to probate and now no monies left in Estate, then the overpayment would just stay on their records as unpaid