Why not contact trading standards or citizens advice, these big companies dont like it when theses organisations get involved, you could also try the advertising standards authority who may help you out.
Throw everything that is available to you at them, tell them you are going to write to your local paper, they hate bed press bad for business!
Also some solicitors will offer a free or low cost consultation to discuss your options, look it up in yellow pages.
I would refuse to discuss the situation on the phone and only enter into written correspondance so that you always have a record of what is been said.
The best piece off advice i can give you is dont let the numptys grind you down, you have to deal with it but be firm and dont give an inch, pay what you consider to be due and then if they want to take you to court they have to prove that you are in the wrong, tell them you will be happy to meet them in court as they have declined your offer off a face to face meeting to iron out this matter, the judge wont look on this favorably.
yours john