We're in the middle of sorting out a similar problem - two weeks ago we got a combined gas and electricity bill for over £1.5k. After picking myself up off the floor I contacted British Gas & asked they could possibly think this was valid for people who have a normal average bill of under 100 quid and was told they don't do any pre checking of amounts. I now have to prove something has gone wrong and that we don't really have a pot farm in the attic!