It appears Energy Watch site has been replaced by a new site- Consumer Direct.
I have had a look and this info is relevant to your case, and if you quote it at them, should be enough for them to reassess your bill.
" I haven�t had a bill for ages � do I still have to pay?
Consumers are legally bound to pay for their utilities. However, the Limitations Act 1980 prevents charges being recovered if the electricity or gas was used more than six years ago. In Scotland this period is five years.
Suppliers should at least offer a payment plan that allows you to repay any debt over the same length of time that it has built up. In other words, if you have not had a bill from your supplier for three years then, when a bill does finally appear, you should be able to spread the repayments over three years. However, if you accept a payment plan then you will not be able to change your supplier until you have cleared the outstanding debt.
However, as a result of the �super-complaint� submitted in April 2005 to the energy regulator, Ofgem, energy companies have been told that, as of July 2006, they should no longer bill for any energy used by consumers more than two years previously where the company themselves has failed to provide a bill. Energy companies are not expected to bill for any amount that dates back longer than a year"
Hope this helps.