We had a similar experience, whereby Carcraft added a warranty onto the vehicle when we stated we did not want it. I suggest you contact Trading Standards and enlist their advice. In the end, we managed to get them to take the car back because we lost all faith in them and didn't want any more to do with them.
You may have a fight on your hands, but if you weren't made aware about the extra �1600 you may have a case. Was it in the small print on the agreement? If so, you may not be able to do anything about it. Wrong, I know, but they're not going to point it out to you obviously. They never do. It's definitely worth a phone call to Trading Standards to find out your rights though. Good luck!