If you check the terms of your agreement you should find that you are able to return the vehicle, providing you have taken reasonable care of it, once you have paid half of the total amount. I believe that this includes any deposit that you paid. But if you have made at least half of the payments you should be ok.
You will need to contact the company that provided the credit in writing and tell them that you wish to terminate the agreement and quote the section of the agreement that states the eary termination terms. They will then probably send you a form out to fill in. It is most important that you continue to make payments until you have returned this form and received an acknowlement from them. Don't just stop the payments or they will say that you have breached the terms of the agreement.
They will then set a date to either come and get the vehicle or they may ask you to take it somewhere, usually a car auction site. The vehicle will then be sold at auction. If there is a short fall on the sale price they have to stand this. However if the car sells for more than the amount required to settle the outstanding balance they must return the excess to you.
I did this a few years ago with a Suzuki Carry van and they sent me a cheque for around £770.
