Hi OUTLANDER
The first thing you need to do is find out what was the cause of the overpayment, if it was an official error then you do not need to pay this back unlike UC where any official errors still have to be repaid.
You do need to seek help from your local Welfare Rights Organisation; you can find your local Welfare Rights Organisation to help you with advice by clicking on the link below and putting in your post code, it will then come up with a MAP with a list of agencies.
advicelocal.uk Gary