If rejection is an avenue you want to explore then the Citizens Advice Consumer Helpline are worth talking to (03454 04 05 06). All of the following is pretty much based on the dealer accepting that there is indeed a fault with the car - if they're not doing that then a written report from a 3rd party professional is probably what you're going to need, but that's uncharted territory for me. Essentially if you bought the car in the last 30 days, then you have the right to rejection and a refund without giving the dealer an opportunity to repair it. After the first 30 days (but inside the first 6 months), the onus is on the dealer to prove that the problem was not present when you bought the car and you have to give them 1 opportunity to repair it. If they fail to resolve the problem on the first attempt, you can then reject the car and demand a refund. If they fail to resolve the problem on the first attempt and you give them a second attempt, and they fail, I was told (by the consumer helpline) that it's harder at this point to justify rejection. If they solve the original problem on the first attempt but another one crops up, then you have to give them 1 opportunity to repair that new problem (and so on). Hope that helps, apologies if not, good luck!