It may also be affected by the day of the month you take your reading. I normally take and submit the readings on the 20th of the month as my quarterly bill is calculated by British Gas a couple of days after the 20th. I too had the heating on all day every day over the Christmas / New Year period, but my 'December' readings were submitted before this began. My 'January' readings, again submitted on the 20th, were higher as these included the usage during the festive period