Contact them, talk to them, do not ignore it or be scared of calling them. They will help you come to an instalment arrangement that will be manageable - they have to or you would have valid reason for complaint and appeal.
I did read recently about overpayments not being able to be recovered by the government, as the UK legal framework did not allow for it. I read that they actually had to pay back money to people who had been made to repay overpayments!
Hope this helps a bit!
Good luck