My cheap snowboard trousers from TK Maxx are awesome. Warm, comfy and at £30 it doesn't matter if I wreck 'em. I chuck my bike tights on underneath them (comfy and no rubbing) then when I'm done I can pop a spare pair of trousers over the top to go home in. The bike tights are ideal ecause you can get changed in a field or behind a car without looking to odd.
I also have a cheap pair of winter boots from Lidl, that I got a while ago. Comfy to board in, warm and supportive - and again it doesn't matter if I trash those either.
Bottom line, wear something warm underneath, with a top layer you don't mind getting dirty and can chck in the wash while you have a hot cup of coffee!