Having worked for Black Horse a number of years ago..I can give you an idea of what will happen.
Black Horse only provides the finance for you to buy the car. If the car has turned out to be faulty, then your issues are with the car dealer. The most Black Horse will do (if your lucky) will talk to the dealer to see if the dealer can do anything to help you. Black Horse are not obliged to do anything to rectify the problems you have. It sounds really harsh..but unfortunately thats the way it is.
Did you buy the car with a waranty? If so, some of the problems may be covered.
If you decide to stop your repayments to Black Horse then this will appear on your credit file as missed payments, and ultimately may lead to you being issued with a defult notice (which will seriously effect your credit file if you do not repay the balance owed) and then finally the car will be repossessed (which doesnt seem like such a bad thing if you are having problems with the car). But if you value your credit rating, then I highly recommend that you continue to make your payments.
Your only real course of action is to pursue the car dealer. If they are not forthcoming, then you could try trading standards or even the small claims court if you have a really strong case.
If the car was purchased on Hire Purchase, then after you have paid (I think) half the repayments then you can VT (Voluntary Terminate) the finance agreement. (read the small print on your finance agreement). This means that Black Horse have to take the vehicle back from you (regardless as to whether you owe more on finance than the actual vehicle is worth) and you are clear of your responsibilities to repay the balance. The obvious downside to this is the fact that the car is no longer yours and you are left with nothing......
I hope this has helped a little........