For fear of repeating myself yet again, you have to give them reasonable opportunity to make good any repairs. That's not my words, that's what the law says, so do that first and if they don't fix the issues then you will have some kind of recourse via the court system. If you march in and demand your money back they are (quite rightly) going to tell you to naff off. Please try searching the interweb for this kind of information, it really is all over the place.
Tbh the amount of people who go to S2K4U, moan about the lack of knowledge AND the crappy customer service and then still go on to buy a car really puzzles me. If someone's being an arse when I'm trying to give them money they're not going to be any better afterwards, so I just wouldn't give them my money in the first place.