Is this a hire purchase agreement (in which case you do not own the car until it is paid off), or is it that you borrowed money to buy a car, which is in your name and owned by you?
If hire purchase, it's amazing they have left it this long, but you will have to settle it somehow if you ever want to own the car. Making payments over a period of time ought to be accepted by them, if you can show them your finances are such that you cannot pay it all at once. You also need to make sure the amount they are demanding is correct. But they might be able to repossess the car if you can't come to an agreement.
If it is not HP, then their next step if you can't reach an agreement to pay over a period (i.e. as above) would be to go to Court for a County Court Judgement. If they get this it would seriously affect your credit rating. However, as annie says there is a statute of limitations which means that they cannot enforce the debt through the Court if 6 years has passed since the last payment or acknowledgement of the debt. If you are very near the 6 years from the last time there is any evidence that you acknowledged the debt, then if you do nothing now you may get to 6 years before they try to go to Court, so they would not be able to enforce the debt. This does not mean the debt disappears - you would still owe it but would be in a stronger position to settle it for a smaller amount.