This is an ongoing case where the dealer admitted to causing the scratches. In that circumstance, yes, you can reject the car. The Consumer Rights Act 2015 theoretically gives you the right to do with within 28 days of purchase. I would guess that the dealer principal is not aware that the repair was sub-standard. Speak directly to him and notify him in writing sent by post office special delivery that you reject the car for a full refund. He will then bend over backwards to remedy the situation.