How do you keep warm in bed?

Up till now, I've been sleeping in an old duvet that I've had for students' years, with a multi-coloured bedspread on top to try and keep the chill out.

Yesterday, I thought I'd take advantage of the January sales, and ended up buying a combination non-allergic 15-tog duvet, which I'm told is the warmest you can get.

So I climbed into bed last night, thought it was a bit chilly but thought the bed would slowly warm up. And ended up dreaming of being trapped in a South Pole arctic base all night.

Did I miss a trick with the tog thing? Is 15-tog the warmest you can get, or am I just living in an exceptionally cold flat?