Under the Consumer Rights Act, you can reject the vehicle and claim a full refund from the dealer if you can prove it was mis-sold, isn’t good enough to drive or has an existing mechanical fault.
You need to make a complaint if you can prove the vehicle isn’t ‘fit for purpose’, of ‘satisfactory quality’ or you believe it has been mis-sold by the dealer.
If there is a fault and you agree for the dealer to carry out a repair, you should agree a date for the fault to be resolved by. You shouldn’t have to pay for any of the repairs and they shouldn’t cause you too much disruption or difficulty.