In reply to andyg:
When in the Scottish hills in winter, we carry a plastic survival bag each and a bothy bag between us. No tent, no sleeping bag, unless we plan to camp.
If we were benighted and couldn't build a snow shelter, we would get into the survival bags and then all huddle together under the bothy bag. Minimum weight, maximum heat, and being together we could help each other through the night.
More important, I would say, is a headtorch each and the ability to navigate in bad weather. I reckon it is far better to be able to get off the hill, rather than endure a night up there unexpectedly. Because once the weather turns bad, you have no idea how long it will be until nice weather comes again.
As for keeping the legs warm, my leg gear varies from a pair of lightweight walking trousers, to lightweight trousers, waterproof overtrousers and thermal leggings, depending on the weather forecast. If you are up in Scotland, the North Cape factory shop does a great price on warm thermals.