I went on maternity in Dec09. In Jan 10 I got a letter from my (very large) employer saying it had overpaid me by £1000 in the last 12 months. I contacted them and said I was on MA (don't work many hours) and they said they would defer re-payment until I went back.
I then contacted them in June10 to say that I wouldn't be going back to work, so could I please arrange re-payment. No-one contacted me so 3 weeks later I tried again and was told that when I handed my written notice in I would be contacted about it then.
Sept10 I handed my written notice in, and I recieved a leavers checklist to complete. On there it said 'any monies owed by employee as a result of overpayment' and my line manager had written no, and signed the form. I assumed it had therefore been deducted from my final salary so I signed the form and sent it back (took a photocopy)
So yesterday I recieve a letter for a legal company demanding that I re-pay the overpayment within 7 days or legal action will be taken. 
Where do I stand with this?