overpaid by employer

[B.Bee]

do i have to pay it back, or where can i find information about it

[Buenchico]

You have to return the money. Failure to do so is an offence under the terms of the Theft Act 1988 (Section 5, Paragraph 4):
http://www.lawteacher.net/Criminal/Property%20 Offences/TA%201968.pdf

Chris

[Thicko2006]

Of course you have to pay it back. If they had underpaid you and you subsequently found out their mistake, would you be happy if they refused to pay you the difference?
Don't be such a slob.

[cesdlv]

The common law doctrine of estoppal might apply, preventing the employer from recovery where:

1) you were in no way responsible for the overpayment;

2) you did not realise there had been an overpayment;

3) you had spent the overpayment.

It is a bit more complicated than that, and you might need to take advice, but all the previous responses are wrong in law.