If you have rejected the car, then you must return it - along with the keys, paperwork, and a letter from you to them explaining that when the V5 comes through, you will be arranging for it to be sent to them.

Keeping possession of goods that you have rejected implies that you MIGHT not actually be rejecting them after all.

If you do take the car back, and they refuse to accept the keys, etc, what then ? Quite simply, take someone with you. They video the refusal to accept, if possible you get the garage to sign a letter refusing to accept, and then you take it back home for safekeeping. Explain to the garage that you will be incurring insurance costs, and will claim for those in the small claims court.

One proviso : you need to be sure that you CAN reject the car. Have you given them the required opportunities to repair ? Have they failed or refused to do so ? Can you show this refusal/failure ? How cheap was the car ? (remember, below a certain price point, faults start to become 'acceptable')

If (as some others have suggested, reading the other thread) it turns out to be something simple, would you be better off taking it to another garage to be fixed, and then billing the selling garage, rather than going down the rejection route ?