Stu is right. Simply return the car and ask for a refund, end of.
When he doesn't, you go to Money Claim On Line (MCOL) and state your case. This is as close to a slam dunk as I've ever seen, especially with the new law in place, but you're never 100% guaranteed a win. Judges can be fickle things.
Keep all correspondence in writing, and preferably either via email or Special Delivery so you have proof of signature.