They need to write everything that has happened and what they want and send it recorded delivery to the car sales place. Referencing the sales of goods act as amended 1979. The car is not of satisfactory quality etc and not fit for use. If you have the original ad etc that is good. Give them a week to respond. It will go from there really. If they still wont budge its solicitors letter and if under 5k, a small claims court, which could take months.
I have recently been fucked over on a car and is in the process of still being sorted.

Speak to your local trading standards and citizens advice

http://www.direct.gov.uk/en/Governme...ghts/DG_183047