he "said sold as seen" which was news to me
Did you read the link, and the link it contained to the Sale of Goods Act?
"Sold as seen" is meaningless b/s here. Either he sold you a roadworthy functioning car, in which case he's liable and must fix it or refund, OR he sold the car for parts or scrap, not as a functioning roadworthy car.
If you don't want to deal with this bloke personally because he is intimidating, he's going to find it easy to keep your money for flogging you a lemon. That is why
he is being intimidating.
You do have other options. First educate yourself about your legal rights. Or talk to a solicitor, legal advice centre, CAB, motoring organisation, trading standards or someone else who knows this stuff.
There may or may not be something seriously and expensively wrong with the Selespeed. Hopefully it will be easily sorted but either way HE OWES YOU for a fault he said was fixed already. Don't let him get away with it.