Hi Glenn
Already some very good advice on here.
I suspect you have suffered a number of under estimated bills in the past. Have you supplied us with readings or had your meter read regularly in the past?
What might have happened is we have now received an up to date reading and this had led to a large catch up bill. This could have wiped off the credit on your account and even put you into debt.
Should this be the case, your Direct Debit will have been reviewed and the payments adjusted accordingly. If there is a debt then the Direct Debit will have been set to cover ongoing usage plus an amount to re-pay the debt.
Give us a call if the new figure does include an amount related to a debt. Ask for the re-payment time to be extended. This will reduce the amount of the monthly payments.
Sorry if this is a bit speculative. Not sure of all the details from what you have posted. However, follow what Jalexa and Mynewt have advised. Make sure you are being billed to actual meter readings; check you are on the right tariff; and look at other suppliers through a comparison site.
Hope this helps a little. Give me a shout if I can help further.
Malc