I got a pair of these: http://www.pecheur.com/en/gb/buy-pants-line-verney-carron-super-pant-28840.html (http://www.pecheur.com/en/gb/buy-pants-line-verney-carron-super-pant-28840.html)
from spain this year. they're a bit too heavy for normal summer trekking, but I can assure you they're well made and tough as old boots. they're not waterproof as such; very heavy and robust twill, with waterproof cordura on knees/thighs and bum. they're so tough it's taken me a couple of months to break them in!
I've found them to be ideal for open-boat (Canadian) canoeing and general "rough country" wear. they are completely impermeable to brambles and gorse.
better still, they're only around the £60 mark, depending on where you can find them. :8)
There are a few Swedish hunters' trousers that are pretty good for the gorse and brambles. Not too much going that cheap but now is maybe the time to look at John Norris at Penrith for sale goods.