Well, I think I may have questioned the high bill myself but I guess most people accept the bill as gospel. Mistake.
FWIW, since I moved to my new place last June British Gas have been suppliers of both Gas and Electricity. They seem to have the Gas bill correct but have constantly cocked up the electricity bill. To my favour, actually, so I ain't saying nowt
I've tried most suppliers, they all seem to be as bad as each other but the most spectacularly bad was Scottish Power (or whatever they're called) absolutely hopeless