In reply to dpomcfc:
Try posting in the gear forum with the title 'Over trousers for scottish winter'. Might get a few more replies.
But to be helful, Marmot Precip full zip pant work well and don't break the bank either! But this year, I'm going to try the Sherpa Lithang Pant. Quite a bit more expensive (but I can get them cheaper) but have a full length zip and an integrated gaiter so you don't have to wear the real thing.
Other good suggestions being the RAB Latok and Neoshell pants, and on the the sallopette side of things, the Mountain Equipment Karakorum.
Hope that helps.