Given your preference for trees, I'm with Dwardo. You don't have to spend anything like fifty quid to get a perfectly usable tent which will take quite a bit of weather. I bought one for a fiver at the end of one of the tent shows, it's seen quite a bit of action in the last three or four years and it's still perfectly watertight. Searches for a photo ... oh, gosh ... five or six years! Here it is at Waddington air show in 2006:
http://www.jubileegroup.co.uk/JOS/misc/waddington_tent.jpg
There's a vent at the top which can be covered over to stop it raining in. In the photo the cover is hanging down the side of the tent to let hot air out of the tent because it was a sunny day. You never know at Waddington.
Condensation will often be an issue with a cheap (usually single skin) tent, I just take a flannel or something to wipe it over as soon as I wake up. If you have a tarp, put the tarp over the tent's door and leave the door open, just close the insect screen. That's my favourite setup, loads of room under the tarp for gear and cooking, condensation virtually eliminated. I use a breathable bivi most of the time even in a tent, so I don't have a problem with water soaking into the foot of my sleeping bag any more.