Thank you for your patience, I am sorry to hear about this. They may be right in terms of their terms and conditions, but you do have some statutory rights under the Consumer Rights Act 2015, on “top” of your contractual rights.
According to the Consumer Rights Act 2015 the car you bought must be “of a satisfactory quality”, “fit for purpose” and “as described”.
Accordingly, you have a right to reject something faulty and you’re entitled to a full refund within 30 days of purchase. I understand that it has been more then 30 days in your case. This means that you no longer can reject the car. However, you can legally return it, provided they are unbale to fix it. I note they are still waiting for the parts, and you are incurring more costs for the petrol etc.
In this case, you could try to receiver this loss from them. Consider sending a letter before claim where you set out your loss i.e. calculate clearly how much each month you end up paying more for the fuel and inform that you will consider taking legal action in the small claims court should they ignore you / fail to comply with the terms of your letter.
If you have to issue a claim in the small claims court, just click here: https://www.gov.uk/make-court-claim-for-money