If you complete an online tax return, one of the last pages asks for your bank acct details in case they have to send you a refund. The government like every other business has been trying to move away from physical payments, as the cost is astronomical. If you’re making a million payments, and the cost of processing a cheque is £2, and a BACS payment is 1p, you really don’t want to send a cheque. (Those aren’t the actual costs- but they’re not a mile out)
I'd move it to a separate account until I got I touch with them to take it back. As tempting as it would be to say nothing and sit on it. If you ever spent it you would be living in fear of a knock at door.
Section 2 theft amendment act 1996, retention of a wrongful credit. Ten years prison. It is a defence to show you took all reasonable measures to return it.