1. Talk to the local trading standards.
2. Read this web page:
CAB Public information site - cars and vehicles
2. Was the car new or second-hand? Was it bought from a trader or a private seller? What were you told about the car when you bought it? These make a difference to your rights.
As a general rule of thumb, the garage (if bought from one) should be given the opportunity to repair the fault before requesting a refund. If they will do neither, you could ask your credit company to look into the matter for you and you will possibly be able to get a refund that way, but check your credit agreement or contact your provider to be sure.