Without loft insulation you could be losing as much as a third of your heating costs through your roof. Insulating your loft is a simple and effective way to reduce your heating bills and you can even do it yourself.
It acts as a blanket, trapping heat rising from the house below. You should also consider lagging your pipes at the same time for optimum efficiency.
For more on installing loft insulation, costs, savings and other considerations see the Energy Saving Trust information page.