I was just looking on 'Magic' and there are a lot of cairns and stuff up there. The nearest circle seemed to be this one
http://www.themodernantiquarian.com/site/1963
but that's over the beck and I guess is really near Hebden not Grassington.
You could look on the 'magic' map if you have a good idea whereabouts Mr Bogg meant (and have a decent internet connection..)