Unfortunately Mercedes-Benz (which probably owns the dealership) is playing hardball and taking them to the full county court over this could cost you £15,000 with absolutely no guarantee of a win. If the car has a major fault such as an engine failure then there would have been no question and the dealer would have refunded you, but rattles alone are probably not sufficient reason to reject a car.

What you could do is notify the dealer in writing (by post office special delivery so it becomes a matter of record) that since he has refused to repair the car, which is not of satisfactory quality, and since he has refused to refund you, you will take the car elsewhere to have it repaired and will use the small claims track of the county court to sue him for the bill. More: www.honestjohn.co.uk/faq/consumer-rights/