I had the same kind of situation a few years back, bought an old Honda Prelude for a few hundred quid from a local garage. I knew the owner of the garage who told me he didn’t know what was up with the car, it would start but would die pretty soon after and wasn’t running properly. Regardless I bought it to fix up but after a few weeks money got tight so I had to move it on.
I advertised it as it was, needed work, a guy came to have a look and was there whilst we spent almost an hour trying to get it to start. Eventually we got it running, he took it for a quick spin and said he’d take it, gave me the money and drove off.
About half an hour later I spotted it broke down outside a nearby petrol station after which the buyer rang me to say he wasn’t happy with the car and wanted his money back, I basically told him he knew of the problems, had been there when we couldn’t start it and had bought it regardless so had accepted the inevitable.
My point is, you told him there was a fault but you couldn’t guarantee what it was, he tested it knowing this and went ahead with the purchase and therefore it is his responsibility. Be firm as legally he hasn’t got a leg to stand on and you were quite clearly in the right.