http://www.direct.gov.uk/en/Environmentandgreenerliving/Energyandwatersaving/Energyandwaterefficiencyinyourhome/DG_064374
Insulate your walls and loft
Is your loft insulation 270 millimetres thick?
More than half the heat lost in a typical home escapes through the walls or the roof. Installing loft and cavity wall insulation will reduce the heat escaping. Combined with a degree of draught exclusion, it could also cut your fuel bills by up to £180 every year.
Cavity wall insulation
Installing cavity wall insulation can take just a couple of hours for a typical three-bedroom house, and can be done from the outside. Your building needs to have cavity walls. You can insulate solid walls too, but this is a bigger and more costly job to do.