In reply to Adam Hughes:
I have moved away from the traditional layering system a little bit, primarily because I am not really a fan of the suit of armour approach of sticking an expensive and relatively heavy layer of Gore-tex (other membranes available) over everything. I find this practice lacking in flexibility, both in terms of movement and temperature control.
The system I now go with is in my opinion more comfortable, more breathable, easier to move in and cheaper as well. Obviously whatever system you use, the amount of insulation you need is dependant on whether you run hot or cold, the temperature and the activity you are doing. What I describe here is great for me on a winter mountain day a degree or two below zero, but can cope with extremis and standing around on cold belays.
For legs softshell is definitely the way forward, I usually wear it on its own, but if it is cold or you feel the cold, layer over a thin thermal. I carry the lightest possible waterproof trouser that I can get over a crampon shod boot for if it gets really crappy and I want extra protection. Cheap is good, breathability does not matter for this item.
For my top half I wear a Patagonia R1 hoody (cheaper stuff can do the same job) next to the skin with a pertex windproof over the top, this is a really warm combination if you are active. I have a 60g fill primaloft pullover which I will throw over the pertex if it is really cold or doing something less active. Finally I have the primaloft belay jacket and very lightweight waterproof in the sac. Again the waterproof doesn’t have to be later most breathable and expensive model, I haven’t worn a waterproof in winter for years, if I am wearing it, I am in rapid retreat. The 60g primaloft is a recent addition, I have managed without in the past, but I am getting soft in my old age.
It is sort of my version of the classic Twight action suit.