As said County Court is where you need to ring, ask for a claim pack, or just do it online, will cost £35, but you can add that to the claim also, if he doesn't respond to the court, you can ask for judgement, which would virtually be automatic in this case, then he gets CCJ, and you have options to get money from him, ie Garnishee order(bank account freeze), take goods etc.
BUT, there is a much simpler way, until paid for in FULL title does NOT become his, and remains yours, ie take the car back drive it away if you still have key etc, or by any other means, and you break no law whatsoever, if he calls plod, just show them paperwork you have, it is a CIVIL matter not legal as it is not theft, you are within your rights to keep all the money he has paid and the car, but might stir him on to pay up.
tabetha