In reply to Nath:
Besides the excellent Simond Alpinism trousers, you might also look at Decathlon's Quechua Forclaz 900 Warm trousers, currently on discount at £40. They're a similar fabric to the Simonds, but with a fluffier inner face, and they're a closer cut, and more 'trouser-like' than the Simonds, which are closer to a salopette.
Double, articulated knees, four zipped pockets, hip belt, shock-corded ankle adjustment. Decent stretch fabric. I'd have preferred a gusset crotch, but the stretch fabric will hopefully mean that isn't a problem.
I've only just picked up a pair so can't comment on longevity or real-world performance yet, but my kit spidey-sense tells me they'll be pretty good.