In reply to Dave the Rave:
Firstly down trousers and top inside a PHD minus will be a very tight squeeze, probably won’t have enough space to loft and will mean you have no wiggle room for those long winter nights.
Personally I think down trousers for UK use a very niche bit of kit, for very occasional conditions. It has to be cold and dry, and you have to want to be outside your tent and static. If you want to wander around outside taking photos, watch the stars then yes. Otherwise in tent, in bag; cook, snooze then sleep.
The simplest way for keeping weight down is a set of thermals and good bag. Otherwise you are just carrying additional packaging around the total weight of down you have with you.
Occasionally I take some synthetic trousers, Montane prisms, doesn’t matter if the hem gets damp ect. Or just thin pertex trousers and top, to put over thermals to go out for a pee and poo in, to cut a little bit of wind and by not being fluffy snow/spindrift doesn’t stick to them. If very dry cold I might take synthetic booties to wander about outside tent without having to get boots from inside my pillow (to stop them freezing), and then fight to put them on/off for a quick pee.
Post edited at 09:14