The 'Mysterious Britain Gazetteer' says that the circle is associated with strange lights. This stone http://www.themodernantiquarian.com/post/30255
is called the Fairy Stone - are the lights the work of fairies or are they 'earthlights'? (some cynics would go for neither, of course.)
The caption to the MBG photograph suggests that the shape of the fairy stone deliberately matches that of Win Hill (SK186850) on the horizon.