How old is your boiler and what make is it?
In Scotland grants are available for loft and cavity wall insulation, not sure about south of the border.
Does your cylinder have an immersion if so you could use electricity to heat the water and how about showers? Electric or off the cylinder? It might be worth getting an electric shower in.
I live by myself in a four bed house and never use any hot water apart from shower, I know the investment in electric shower would outweigh heating up the cylinder.
I fitted a radiator in my garage to keep the car warm :-)