To be honest, I'm dubious about skimping on something like this. OK, they cost £40 but from the sound of your post you're going to be doing serious stuff.
Whilst I'm not an outdoor expert, i do do a lot of outdoorsy stuff like hiking and walking, and I can tell you now that there are certain things you will not want to skimp on in terms of cost; boots, for example. It might sound silly, but the more expensive trousers are likely to have better features such as being quick drying, water repellant to keep you dry if its raining, and so on, that cheaper trousers won't have. Some have thigns like built in mosquito repellant and all kinds of things, besides being generally lighter, more breathable, adn so on.
In short, you may well wish to consider spending a little bit more money on these kind of things. You very much get what you pay for!
Myu suggestion would be to go to a larger outdoor store such as Cotswold or Blacks, and see what they have on sale. In one of my local shops, for example, tehre are some nice £50 Hagloffs trousers reduced to £22, which had I got money I would ahve bought! In short, you might want to look at going down a middle route, as specialised outdoors clothing will be a lot more suitable and on teh whole less hastle than thing sfrom M&S and so on.