I think you're safe. He would need to either accuse you of selling an unroadworthy car and prove it or take you to court for breach of contract. Before he can accuse you of selling an unroadworthy car, he would need to email or send you a copy of the MoT failure form.

There are other factors to consider too: whether or not the man was a dealer (whether or not he had bought and sold any other cars during the last 6 months: an important question in court), in which case he was required to make a professional judgement of the car before he bought it. And the price he actually paid. If he paid substantially less than the retail price, then he could only expect to get a car worth substantially less.