They did the same to me almost 12 months after leaving - I ignored them for a while but ended up agreeing to paying a nominal amount for a long time! Seems that if it is a Public Debt they will go to court.
I took advice from a firm that provide my current Company employees with legal advice and although I argued that I had received the overpayment and had spent it not realising in the flurry of payments that are terminal gratuity and final pay that I had been overpaid by about £300. I was strongly advised that I should agree a settlement as the law was on the MODs side despite what I may have read on similar cases on the Internet. I accept that it was an overpayment and that I should have noticed...however, it still grips you when after 30 years of unpaid overtime and countless dets in gruesome conditions some spotty civil servant chases you for a public debt. Who owes the debt
Ed