Hi emgem23
I'm sorry to hear of the problems you are experiencing...I am sure you find it quite exasperating to say the least.
Your best course of action IMHO is to get yourself some advice from CAB or maybe you have access to a free legal advice helpline through some insurance you have for your home or car? There are obviously certain conditions that apply to HP agreements which I am not totally sure about, but in any event these are in place to protect your rights as well as those of the finance company. I feel sure that there are terms in there about the goods being "suitable for purpose" or being of "merchantable quality" and being able to reject them within a set timescale, particularly with a HP agreement rather than a normal unsecured credit agreement.
What I will emphasise to you is this...even though you are in this situation, it is important that you keep to your side of the agreement by making payments as per the terms. By not doing so then you begin to open the preverbial can of worms and before you know it you're having action taken against you and possibly the car being taken from you. It also shows you in a better light than the finance company should you need to start any legalities.
Years and years ago I had an issue with my bank not honouring my claim for payment protection insurance. I saw a solicitor who drafted me a letter for about £40 (probably a bit more now, but still a lot less than the settlement figures you're quoting!) and needless to say I had my claim approved by return. Sometimes it's just about taking the right action and beating them with the same stick that they would have no hesitation in beating you with should the situation be reversed. The key is to do it properly and with a bit of help with the jargon!
I hope that it turns out well for you in the end, it would be great to hear of your success in due course. Best of luck.