You can try, however, I don't envision much success if they're already denying that it's faulty. If the seller refuses to accept the vehicle rejection then you'll either need to seek mediation with an alternative dispute resolution (ADR) service or take legal action against the dealer. Court should be a last resort as it's expensive with no guarantee of success. I would advise speaking to The Motor Ombudsman and Citizens Advice. They may advise that you need to get the issue investigated by someone independent to gauge whether it's a fault or characteristic of the car in order to process with action against the dealer.