If a defect becomes apparent within the first 30 days then you have a right to reject the item and to insist on a refund.
If a defect becomes apparent within the first six months then you have a right to insist upon a repair and then if the repair fails, you can insist on a refund.
You assert these rights by sending a letter making it clear that you are asserting the right. However, – especially with car dealers – don't expect them to be compliant. But send the letter anyway.
Even if you aren't sure that you want to reject the item and that maybe you'd rather repair, send the letter anyway so that your position is reserved.
Your letter asserting your right under the Consumer Rights Act should be sent by first class recorded delivery – and also by email as well – but make sure you have evidence that you did assert the right.