Much as I like the idea of a no-comeback deal, it being one decided between the two parties alone, no vendor can opt out of the Sale of Goods Act and related legislation. So whatever they say about trade terms, no comeback, sold as seen, etc etc, if you are a private customer buying a car worth (I think this is the cut-off) £2000 or more, any trader telling you any of the above is seeking to avoid his liabilities under the law.
I am currently having a fight about something sold to me that was defective from the first time it was used, and the amount of deliberate attempts to evade liability by the retailer is both irritating and depressing. My experience last year of buying lots of kit for a house being redeveloped is simple - they only want to know you just before you pay for it, and faults afterwards, tough, at which point you get treated as though you have threatened their first born by seeking your rights, however politely. Moral of story, deal with someone who has a reputation they wish to protect.