In reply to McDuck:
I wear a North face salopettes that I purchased in 2002 (dont know their name) they are quite heavy but work well. They have full length zips which I open fully when walking in and out. I wear nothing underneath other than my underpants.
I personally find that I dont think too much for layering on my lower half in the UK. Its doesn't get cold enough for long enough to impact my legs, even when sitting on a belay for an hour or so. I layer the top half, wear good socks on my feet with good boots and my legs just get what ever I feel like wearing on the day.
The salopettes with nothing underneath work well for me. My other choice is low alpine base layer tights (£10) and a light waterproof trouser over the top (£7 Karrimor) works a treat, and has never leaked, or not been warm enough.....So far!!
Go with what your are comfortable with. Trial and error came up with my current methods I have technical soft shell trousers costing the best part of £100 and I never wear them.