In reply to Chris Davids:
> How about a RAB Quantum 600. >
I've got one, when I first used it I was freezing
and furious, I'd spent so much money and I'm not a cold sleeper (I've been OK to -15 in a 3 season Vango. Anyway I'd been using a thermarest guide light (or some redish thing with it) so I went back to the good old green therma rest, thank god I didn't ebay it) and I'm toasty again.
So in reply to MattyK I would suggest it may be more or just as important to think about what you're lying on.