And I will tell you - insulation, insulation, insulation.
I don't normally endorse commercial organizations but you could do worse for the planet than head to B&Q and pick up some subsidized loft insulation. A quid a roll means you can lag your loft for around £25 - £30. Just don't try to get eight triple-roll packs into a hatchback like I did. (There are some things you can't do on a bike).