In reply to Reader_Rambles:
Try the Decathlon/Simond "Men’s Mountaineering Trousers - ALPINISM LIGHT EVO - GREY". £60ish, but very light and great freedom of movement, quickdrying, windproof but breathable. I use them all year around for trad, indoor climbing, bouldering, walking and Scottish winter (with waterproof overtrousers).
I've also used ME Ibex, but they're cut a bit long and not as comfy/free around the crotch whilst climbing as the Simond ones.
I think other brands, e.g. Montane and Rab, will all have their version. Softshell is much better than jeans or cotton IMO, because it is so breathable, light and quickdrying.
Otherwise, some acrylic/elastine tracksuit bottoms, if you just want something cheap.
Post edited at 11:05