I keep my bedroom quite cold, and sleep on a mattress with a duvet. I find it helps to have the duvet underneath me as well as on top; I guess the duvet is better insulation than the mattress itself.

That reminds me: I've been meaning to get a proper hot water bottle for ages. The one I use contains high-tech jelly that you heat in a microwave for 5 minutes. It's really good, but it won't work if power is cut. I need a lo-tech water one as backup, because I have ways of heating water that don't need electricity.