This cache is not on the ancient Forest boundary of the 1240 Perambulation, but has been placed as a waymark between Wallebrokeshede and Cator Common. More details about the Perambulation can be found on the cache page for the Dartmoor Forest Bonus. On the front page of the logbook is an adjustment to be made to the Bonus co-ordinates, which you will need to find the Bonus.
If doing this cache as a standalone, there is parking at the nearby pub (waypoint below). Angle down the hill from the pub towards the brook, and then follow the brook, keeping it on your left. There are some wet bits, but the going is not too bad.
If doing the Perambulation as a route, the next point on is Cator Common. From this cache at Soussons Down, backtrack to a breach in the wall and continue down the brook between the wall and the fence. When you near a gate by the clapper bridge at Runnage Farm (waypoint below), you will need to cross the wall, then climb over the gate, which has conveniently been padlocked shut.
Continue towards the road, keeping close to the wall where it is slightly less boggy. When you reach the road, cross the road bridge and follow the side road west along the brook towards Pizwell Farm. At the farm, bear left and recross the brook on some stepping stones, or use the flimsy bridge a little way downstream. Continue on the track until you reach a gate (waypoint below) and go through this onto Cator Common. Follow your arrow across the common, keeping slightly left of the straight line to avoid the worst of the mire.
This cache site is liable to flooding. Please make sure you replace the cache with the really heavy rock on top so that the risk of it getting washed away is kept to a minimum.