I think as a basic guide just go a long way the opposite direction form anything touristy!
Just a a guide A friend and I went camping for a couple of nights in Thetford Forrest back in the late 80's, it ended up a near disater, but that's another story!
we chose a place waaay off any trails...near a river.
we had an open fire, tent, collected rainwater on a huge tarp, i was getting fire wood turned around and 10ft from me was a red deer stag with huge antlers! I guess the smell of the fire and my friend mucking about in the distance masked my scent/noise.
we were camped here (roughly at the white square in between the forestry blocks)
zoom out you'll see how close it is to the town, well within walking distance from the train station.
http://maps.google.co.uk/maps?hl=en&ie=UTF8&ll=52.441179,0.720849&spn=0.012766,0.027595&t=h&z=15
we camped there before the duel A11 carriageway was built (to the east).
basically we only saw one person at a distance.
we were set up about 10 meters back into the forestry block
we could have stayed there for days, without seeing anyone.
once you get past recent footprints and onto paths that have branches across them and grass at shin level you know you won't see many people
(It is not a public wood as far as I know)
I'm not saying you should...I'm saying you could