In reply to medomjones:
I've got a snugpak osprey or softie 12 (can't remember which of the top of my head), got it for £90 online, never been cold in it, wet or dry, I'm 6'2" and have plenty of room. Downside is it's a tad heavy and bulky when packed.
Compare this to my mates alpkit down bag (whatever the standard warm one is called...1200 pipedream or something), in Scotland this January, even with a bivi bag, in a tent, his was soaked due to the high humidity after 1 night, and he was freezing even in all his clothes. Mine got damp but dried quickly in the day (being synthetic) and kept me warm and smug all week.