just a blagger and you should NEVER have goven him £50. Tell him to piss off.
He inspected the car, was happy to buy it anything that happens after that is down to him unless the car was stolen or something.
I once had a guy call me a few days after i bought a car off him to say that the cash I gave him (that we BOTH counted) was £200 down and could I do the right thing and send him £200 lol!!
I told him I would, do the right thing that is and send him fook all.