Might be worth double-checking that they have you on the correct VAT band as well - I was paying the higher business rate for about a year before I noticed. Got a decent rebate from them after they recalculated it at the much lower domestic rate. Something like 17.5% vs about 5.5%, I think.

Thanks for the warning, but they've charged me 5% VAT, so that looks fine.

After double-checking their calculations, it seems I really do owe them £500, because I've been using £50 of electricity per month for over two years and they've been charging me amounts varying from £20 to £40 for most of that time. Oh well, such is life.