Go to your nearest CAB for advice or consider getting copy of the Welfare Benefits and Tax Credits handbook from CPAG for about £9 (it is normally £32 but if you are a claimant you get it for about £9) - they have a website www.cpag.org.uk(posted 7073 days ago)This is a complex matter and "common law" allows them to recover the money via the courts and that's where you need to find out if such attempts are statute barred. But even so, they can only recover overpayments under certain circumstances ie: where you misrepresented info, failed to report a change in circumstances etc.
Best to seek proper advice asap.
Good luck
Moira