1. Well I'm not sure what the rules are in the UK, but in Australia you have a 3 day "cooling off" period, which means, after you've bought the second-hand car (or laid down a deposit) the person/business you've bought it from is required by law to refund your money should you decide you don't want it. After that 3 day period, I'm not sure there's anything the law can do to help you: it's between you and the garage you bought it from (or laid down the deposit with). Was there actually a contract involved though? If there wasn't you probably have a pretty good chance of getting your money refunded (if there was no official contract then he hasn't really got a leg to stand on - though I'm sure there's some principle that he could take you to court for if you did back-out of the deal).

I'll dig out my Legal Studies book tomorrow, there's a whole chapter on used cars (and I presume the Aussie and English laws would be relatively similar).

As for number 2, sorry mate. NFI.