95% of a incadesant lamp energy dissapates in heat so in the winter yer house is cold and your heating has to make up for the 95 watt of heat lost from a 100w lamp
True, but a boiler is more efficient at producing heat and a unit of gas is cheaper than a unit of electricity.
The best thing to do is to insulate and draft proof your house. That'll more than compensate for the loss of heating by your lighting.
Also, turning your thermostat down a degree or two will save you a packet.
I shaved around 15% off the number of units of energy I use by turning unused appliances and lights off, turning the thermostat down and slightly shortening the period of time the heating is on.
More recently i have been fitting loe enegy lamps. I get mine in the Range for £0.99. 14w are very bright.