Check your statement too: now that rates are falling, these over-estimates and credits are potentially rich pickings for the wily energy companies - even supposedly ethical ones like Good Energy!
Update: 21 February 2009 After I emailed Good Energy, asking for a correction to my bill, they replied really quickly and a couple of days later a new statement has arrived. The estimate to the end of January has been adjusted in accordance with the actual reading in early February, bringing the bill down by over £13 (roughly 10% lower)! This was definitely worth querying, and you should do so too.