The bottom line is if you do nothing about trying to return the windfall into your account, you could end up with a criminal record. I therefore strongly recommend you contact your bank and alert them to what has happened and ask them to cancel the credit.

You have been very candid about how you were not expecting a windfall payment such as a Christmas bonus. However, you may wish to note that an exception to the legal rule about having to return funds which land accidentally in an account is in circumstances where the recipient can argue, with credibility, that either they did not realise there had been a mistaken payment or they were expecting a payment and so relied upon the funds received being theirs. 

An example of where a person may not realise a mistaken payment has been made is an employer paying an incorrect enhanced salary for a number of weeks or months. In that case an employee may simply think they had received an incremental pay rise and use that as an argument to retain the extra payments received. Though no doubt if successfully argued an employer would take it into account at the next appraisal and salary review.

An example of where a person may say they were expecting a payment and replied upon it, is where a bank credits a customer’s account and the customer has no reason to believe the credit balance shown is incorrect and so proceeds to use the money to, say, pay off debts or otherwise rearrange their financial affairs. It was an argument successfully used by a customer against Lloyds Bank in 1950. As I say, any such argument must be credible and the threat of the Theft Act hangs over any person behaving in that manner. 

I do think again of the Victorian Mrs Solari, who it is understood genuinely did not know her husband’s life policy had lapsed. Had she spent promptly the money paid to her in error, she may have had a legal argument grounded in the more sophisticated interpretation of the law set out in the Lloyds Bank case (heard over 100 years later) not to have to repay the money paid to her by mistake.

Your own situation is different. You certainly have been unjustly enriched. You know the credit to your account was mistaken and you should take steps to return it forthwith. That said, do ask the bank to make a discretionary award to you for your time spent in sorting out this mess not of your making. Such an award would be a windfall you may retain.

Ask a Lawyer is written by Gary Rycroft, solicitor at Joseph A Jones & Co, and published twice a month on Mondays. Email your questions to askalawyer@telegraph.co.uk