We are changing power supplier and have been told we are 'high users', using enough for 6 when in fact mostly there are only 2 of us in the house! We have had a new condensing boiler, have double glazing throughout the house, thermostatic valves on all the radiators and loft lagging. How can you tell if you have cavity wall insulation, as that's the only other thing to check? The house was built in 1953.
Oh it makes a difference alright.
But how much insulation (and of what material) have you got in the roofspace? This (and the windows - which you have dealt with) are where the largest impact occurs.