Mine or Not?

[ajmant]

A friend of mine has been sent a cheque by his Accountants for a refund of tax for the year ended 5 April 2004.  The amount is for �1200. 

However, this is the first he has heard of it and the firm has not to my knowledge completed any form for him for that year.  He has in the past received letters from his accountants regarding another client with an identical name and we both believe that this cheque was sent in error.

I believe that he should call the accountants to query the amount received.  However, he has already cashed the cheque and started to spend saying its their problem. 

Is he going to have legal problems if it transpires that the money was not his and the money is asked to be returned?

Or are the accountancy firm going to have to admit to the error and admit defeat in getting it back?  Can anybody help with the legal situation here?