This thread has been running for 4 weeks and the car was 6 weeks old at the start - sort yourself out with the rejection letter NOW, the longer you ponder the less your chances of a full refund or replacement car.
Do it !
Absolutely!
You have rights as a consumer. Don't let them talk warranty issues. They sold you a product that is not right! The seller has the responsibility to sort it out. If you have given them 3 chances and they have failed to rectify a problem then you can and it your case should reject the car as not of satisfactory quality / not as described (it didn't say in advert- would break every week, did it?)
Send the letter now.
If that helps:
I am writing to you with regard to my new xxx which I purchased from xxx on xxx.
Immediately after taking delivery I noticed several faults which are detailed below. The car was returned to the service department on xxx to attend to these matters.
The faults remain which are of great concern to me on a brand new vehicle for this price which should be perfect.
Firstly, there are numerous xxx.
I’m sure you will agree that this situation is unacceptable on a new vehicle. It is of great concern to me that this could be the beginning of a fault that can only get worse over time.
I am willing to give your company one more chance to resolve these problems in full and permanently such that they do not re-occur. I am sure that you will agree that such problems are unacceptable and disappointing in a brand new car.
The Supply of Goods Act 1994 requires that a vehicle 'be of satisfactory quality, having regard to its age and price paid.'
If another attempt to fully and finally correct the faults detailed above is not successful I will reject the vehicle and demand either a replacement car to the same specification or a full refund.
I look forward to your early reply in writing, within a maximum of seven days.