Section 1 of the Theft Act 1968 states:
"A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it".
Section 4 of the Act states:
"�Property� includes money"
Section 3 states:
"Any assumption by a person of the rights of an owner amounts to an appropriation, and this includes, where he has come by the property (innocently or not) without stealing it, any later assumption of a right to it by keeping or dealing with it as owner"
Section 5 states:
"Where a person gets property by another�s mistake, and is under an obligation to make restoration (in whole or in part) of the property or its proceeds or of the value thereof, then to the extent of that obligation the property or proceeds shall be regarded (as against him) as belonging to the person entitled to restoration, and an intention not to make restoration shall be regarded accordingly as an intention to deprive that person of the property or proceeds"
http://www.statutelaw.gov.uk/legResults.aspx?L egType=All%20Primary&PageNumber=2&BrowseLetter =T&NavFrom=1&activeTextDocId=1204238
Putting that lot together means that, because you failed to refund the money as soon as you realised it was paid into your account in error,
you have already committed theft. The maximum penalty for the offence is 7 years imprisonment. If I was you, I'd hand it back as soon as possible!
Chris