Just as a follow up to my previous post, i had a look and British Gas HAVE signed up to the code of practice.
Thought this little directly quoted excerpt might be useful.
"Five of the main energy supply companies in the UK have signed up to the Billing Code, namely British Gas, npower, EDF Energy, E.ON Powergen and Scottish Power. The Billing Code provides consumers with additional protection over and above that required by the regulator Ofgem for accurate and informative billing.
The Code includes a requirement for consumers to be protected from debt where the supplier is at fault for not billing energy supply. Where the supplier is at fault, consumers cannot be back-billed beyond 12 months from the date on any subsequent bill."
That is a direct quote from the Energy retail associations code of practice. So whilst they can make you pay for the year, even if it is their fault, they cant go back beyond this. Get them to work out how much this is- ensuring it is all at the relevant rates, not todays higher rates.
Then work out how much you can realistically pay back each month. Make them an offer of paying this back in those installments, if they refuse, say you will contact Energy watch. My guess is they will bite your hand off to accept if you follow this approach.
Here is the link to the code, it is full of useful info,
http://www.energy-retail.org.uk/customerbillin g.html