That's incorrect, even if a dealer says "sold as seen" it means sweet FA - terms like these don't exempt them from their legal obligations. (Private sales are a lot less protective of the buyer but that doesn't matter here).
You may want to seek free legal advice, but I'm pretty sure that the car isn't as advertised (although probably still fit for purpose) so you should be entitled to a refund, although the garage will probably (begrudgingly) offer a replacement before that, after a bit of arseing around... at the end of the day they should have HPI'd the car themselves before purchasing it or excepting it as px so should have some form of protection themselves to recover their loss.
I'd just turn up and see what they offer, you might be pleasantly surprised, and if you're not you can just say you'll think it over and leave and then research some more - the Sale of Goods Act 1979 is your friend here.
There's a fairly good article on Autotrader here:
http://www.autotrader.co.uk/advice/2010 ... ning-a-car