I would say get some of those craghopper boyos. normally filthy cheap at TK Maxx and they do well for climbing as well.
I do like the moon Cyphers for climbing in. Much more durable than craghopper and have a good crotch action (woof)
Ronan will tell you to go to primark and get strechy jeans but this is a man who wears jeggins so do not trust him