Have you tried UK customer options on 0800 800 030, they should be able to sort this out.
It sounds like your contract period ended, and you were paying the full amount. Customer options should be able to correct that, you may even end up paying less.
I am sorry to hear about all the problems you have had getting your bill corrected. I'll be able to take a look at this for you. Please can you send me in your details using the "Contact The Mods" link found in my profile?