Average winter figures for heating a house are not available on the internet. This is probably because it varies so much depending on the temperature, and also figures tend to be available for different types of fuel rather than the total cost of heating. However, if you contact a consumer organisation such as Which? they should be able to help you find more exact figures.
Some information that may help: With the recent price rises, as of November 2012 the average cost of heating a UK house (3 bedrooms is typical anyway) with dual fuel for a year is now £1,318. This is an increase of 6%, on top of many other increases in recent years. So the cost of winter heating would be at least a third of the total annual amount, and could be as much as half - it also depends on how severe the weather is during spring and autumn.