what if you kept quiet and they never asked for it. Is there a cut off period, after which its yours?
I was off sick 4 years ago for 3 months, during which time I was on full pay. when I went back to work, they also paid me over £900 statutory sick pay. I was tempted to keep quiet, but knew that if I spent it then found out they wanted it back I would be buggered. It didnt seem right, cos I'd been on full pay, so I asked...unfortunately I was right, so they took it back the following month.
A few years ago my wife noticed that Amex had been taking monthly payments from her account...she didnt have an Amex card! Turns out whoever input the genuine customers direct debit details got a digit wrong!
I had fun telling her bank that they had to repay it due to the direct debit guarantee.