i am having similar problem with prepaid method - changed supplier from BG and then received a 'final bill' for £250! asked how this could be as i 'prepay' for my gas. informed i'd be taken to court if i don't pay. again asked for breakdown of costs. was then informed that £120 was being credited to me as i had had my payments mistakenly paid to another account and also my new providers had not given BG a up to date meter reading therefore it had been estimated - my new provider had provided the correct reading on two occasions! still waiting for reply from BG