In reply to Legs:
Charterhouse mines / Ubley Warren does have potential to get impressively disoriented on a dark night but its a small area and you'd need a large scale orienteering map to make the most of it.
Black Down and Beacon Batch is the biggest slice of open moorland, certainly enough scope for a couple of hours plot-and-bashing if you're creative. There's scrubby vegetation over the top which makes it hard going off piste but the north slopes stay relatively clear until the bracken comes up.
Paths and RoWs marked on the 1:25 OS mostly do exist on the ground but some quite vague and many are boggy. Not marked are the lines of anti-invasion hummocks marching over the plateau, or the bunker by the gate at the top of the access track from Tynings.
Lots of path junctions to set as waypoints, trig point at the top and the twin brooks for potential parallel error pitfalls. Watch out for the ponies