I suggest you speak to Citizens Advice ASAP - your documents from the sale should show how much the car you bought is, how much your part exchange was, how much you are paying for credit and so on - otherwise how do you know how much you owe? You may also be able to get far cheaper credit elsewhere - All this should be laid out clearly. Take with you everything you have relating to this when you go to your local Citizens Advice (they are in the phone book or http://www.citizensadvice.org.uk/ ).
Also - if they have already approved the finance (which it sounds as if they have from what you say) then why are they seeking further information? Ask. I ALWAYS challenge when I suspect information trawling and ask for a proper reason when companies ask for more info before I give it - and if I'm not satisfied they don't get it. It's amazing how many times it has turned out that they don't really need it ! Usually you are asked to prove income before a finance company agrees to lend to you, not after!
It sounds that as the car did not have a full service history, but you were told it had then you've been misled - which I would be fairly confident would nullify the agreement if that is what you wish - personaly, if things are as you describe, I'd be trying to cancel this agreement as it sounds very irregular from what you say.
Phone or go to Citizens Advice (tomorrow!), Take all the documents you have with you (it helps to write things out in the order they happened - i.e your letter to them etc, as it makes it much easier for someone else to understand as fully as they need to if they are to help you). The main thing is to ACT QUICKLY as credit agreements are sbject to certain time limits (though if you have really been misled into making an agreement then it has no validity anyway) .
Any letters you send to the company - use recorded delivery and keep a copy of your letter. You should certainly be told in writing how much the credit agreement is for, how much your part ex was etc. i
It seems as if you may have a right to claim this agreement has no validity as you were misled (if thats what you want) or else may be in a position to negotiate a discount on account of the lack of service history you were promised if you still want to keep the car. It certainly doesnt sound as if the required Credit Agreement information has been given to you? Again it's hard to say without seeing what you have - so see Citziens Advice ASAP.
Let us know how you get on! :-)
Do let us know how you get on. :)