Actually it doesn’t. The border continues in an almost straight line following the line of initial approach passing just to the right (East) of Jenny Storie’s stone. The fence turns at a fair angle to the right toward the summit so a small area to the NW of the fence is still in England, At its closest the border is approx 100m NW of the summit.
When I activated SB-004 from GM, I has to use my GPS with the border plotted, to ensure I really was in GM.
Many of the depressions in the summit are not natural but were create by explosive many years ago to allow certain vegetation to grow.