In reply to Tig Smith: I'm quite a warm sleeping person but was in a -20 bag and toasty. One thing we found important though (especially when bivvying out) was sleeping on a karrimat and a thermarest (ultralite in my case). This made a huge difference - on the couple of nights I slept on only one or the other I got cold patches through the bottom of my sleeping bag. We were sleeping on ice every night though.
Ref clothing, most of the time in the day I wandered around in a baselayer and a polartec powershield windproof fleece & schoeller trousers with occasional polartec trous underneath. Belay jacket for sitting in for breakfast. Night-time (when we did most climbing due to better snow/ice conditions) was usually baselayer and montane extreme pile/pertex with daytime trouser combo (you'd probably be fine with a down jacket on top of daytime fleece, just be careful not to sweat too much, I did once, bad idea). It never rained with us so windproofing and breathability of clothing was more important than waterproofs.