Thank you for confirming, so the contract states until the full payment is made the car is not yours but the seller took the car before you could make the full payment. He therefore owes you a full refund of the money paid thus far. I recommend you write to them and ask them to refund the money owing within 14 days or face legal action.
You will need to register at http://www.moneyclaim.gov.uk so that you are ready to issue the claim in the event they dispute the claim and do not pay you. The website is very user-friendly and you would not need a lawyer to use the money claim site. Claims with a value of under £10,000 are classed as a "small claim", so legal costs are not recoverable and the matter may be dealt with on paper by a Judge, not at a hearing. A hearing may be necessary if the court thinks that oral evidence is required to dispose of the case.
You would claim the sum for the loss, the court issue fee (details of fees are here at page 5: http://www.gov.uk/make-court-claim-for-money/court-fees) and court interest which is 8% calculated on a daily rate from the date of loss to date of court judgment. The site allows you to calculate the interest and add it to the claim.
If you are on a low income or have low savings (or in receipt of benefits), you can ask the court for a fee remission so you do not have to pay the court issue fee.
If you win then once you have the CCJ from the court the defendant has 14 days to pay in full. If they do not then it gets registered with the credit agencies after 30 days. You can also enforce the CCJ with the county court bailiffs or transfer the debt to the High Court for a small additional fee assuming the total amount owed is at least £600 and you can use the high court enforcement officers who have greater powers than county court bailiffs. The transfer fee is added on to the debt and payable by the defendant.
I trust this assists you in understanding your position however, should you have any supplementary inquiries, do not hesitate to ask and I will look to clarify or expand on my previous response.