Hi KM.Carter,
Sorry to hear about your trouble with BG but honestly I am not suprised but I know thats not what you want to hear.
Firstly there are new rules concerning the back billing of accounts, and from 1st July 2007, energy companies are only allowed to back bill you for a period of 12 months. However this rule doesnt cover, i dont think and sure will be corrected (!) it covers they have to keep your statements for a period of 12 months, realistically as you get your bills quarterly, as you pay quarterly, you really should keep them yourselves (something I am sure you may be told from BG).
However I find it also strange that your past accounts have also been wiped from your internet account. I am with ScottishPower on their online account. You can see every statement ever sent you, whether estimated, then replaced with an amended statement based upon your own reading for over 18 months.
First thing first I would see if you have kept any of your invoices, clearly, they would have had a giroslip on them and you would have either detached those to pay, or paid online.
I would also explore the point of you mentioning you have been keeping your own readings, do those that you have taken reflect such a massive jump? or on a more common level, for example if you have an E7 meter which has two readings, day and night, have they in the past tranposed these readings, thus charging your day time usage at your night time price, as usually when this is realised and corrected the bills do leap up.
If you cant realistically get any answers to the above then I would contact their customer services again, and ask them for a statement of account, usually you may find this might be chargeable, but will give you a full breakdown of payments you have made against consumption and see if it matches up with your records. Ask to speak to a customer service manager or ask for their complaints proceedure and mention you intend to go to Energywatch, this usually *** the ears up of a customer service advisor.
Best of luck and let us know how you get on