This cache is the 17th point on the Dartmoor Forest boundary, as established by the 1240 Perambulation. 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.
The next waypoint on the Perambulation is Eastern Whittaburrow. You will have seen the summit as you walked down the Wella Brook, looking like a submarine conning tower on the skyline. It could be reached by crossing the Avon at this cache, if the river is low enough, then climbing straight up the hill. There is a slightly easier route, though, which involves detouring upriver to a bridge (waypoint below) then heading diagonally up the hill.
Neither route is particularly easy, as there is no track (well I couldn’t find one), the grassland is very rough, and the hill is a quite steep here.