Might check to see if previous bills were estimates, if so they recalculate after a meter reading. That is when the numbers jump high.
I came back from a long holiday July 06 to recieve an electricity bill stating we had a debt of over £3,300, despite paying by DD every month. What a palava! They had not read the meter for ages, took a reading and readjusted months of bills to reach that sum.
I won't go into much detail, as it was extraordinarily tedious, but it took me six weeks to get them to decrease the debit. Last August, I noticed a debit on the bill that was much higher than I made it, asked the electricity company to readjust it. Bang! Suddenly they returned the previous debt, which had been reduced to under two hundred pounds, and we had nearly £3000 in debt again. Took me until last month to get it erased. Hours studying bills, hours convincing various customer service people it was incorrect, as they will not take your word for it-you have to prove it, and months of requesting yet another readjustment. We had a new meter installed August 06, which I thought would make it easy to fiqure out as the meter was pretty much at zero. Oh, no, never so simple. After three readjustments and ignoring my question as to why I had three different meter readings, by them, for the same meter on the same day, all increasingly higher. Finally, after threatening to go to Ofcom/ombudsman, a kind lady spend two months working on our case. Blamed the gent who reduced the original high debt, and said the initial readings were incorrect. Finally, she managed to wipe all but £430 off the debt, which was fair enough as the first years direct debit was really low.
I sincerely hope this is not the case with you, as it is a right tedious pain in the bottom to rectify.
I know of a few people who have had similar trouble with both gas and electricity companies.
rant over