Theft Act 1968:
Dishonestly retaining a wrongful credit.
(1)A person is guilty of an offence if—
(a)a wrongful credit has been made to an account kept by him or in respect of which he has any right or interest;
(b)he knows or believes that the credit is wrongful; and
(c)he dishonestly fails to take such steps as are reasonable in the circumstances to secure that the credit is cancelled
Did he dishonestly fail to take reasonable steps? From the article, it seems that the bank asked him if they could take the money back. It would seem to me to be a reasonable step for him to say "yes". Did he dishonestly do so? Tricky one; Theft Act 1968 specifies what dishonesty is not, leaving it open to a jury to decide what it is.
Theft Act 1968 says the following cases are NOT dishonesty:
(a) if he appropriates the property in the belief that he has in law the right to deprive the other of it, on behalf of himself or of a third person; or
(b) if he appropriates the property in the belief that he would have the other’s consent if the other knew of the appropriation and the circumstances of it; or
(c) (except where the property came to him as trustee or personal representative) if he appropriates the property in the belief that the person to whom the property belongs cannot be discovered by taking reasonable steps.
None of those applies, so it's definitely possible that he did act "dishonestly" under the Theft Act 1968, in which case yes, this
is theft.
HOWEVER, there is an incomplete definition of "wrongful credit" here that could throw a spanner in the works:
A credit to an account is wrongful to the extent that it derives from—
(a)theft;
(b)blackmail;
(c)fraud (contrary to section 1 of the Fraud Act 2006); or
(d)stolen goods.
One legal interpretation of "to the extent that" is simply "if", in which case this was NOT a wrongful credit to an account, and this entire section of the Theft Act 1968 ceases to apply.
HOWEVER, it's not "if and only if"; could one argue that this was a wrongful credit? Well, it was certainly wrong in that the account that was credited was not the intended account. I suspect a good lawyer could reasonably argue this, but I don't need to go looking for case law; in this case, the court agreed that the money must be returned, which I think is itself case law supporting the idea that the recipient was in the wrong.