Here are a few to be going on with - there are doubtless heaps more:
http://www.themodernantiquarian.com/site/462
King stone, rollrights - this may be the one you're thinking of?
http://www.themodernantiquarian.com/site/2092
wimblestone
http://www.themodernantiquarian.com/site/104
maen llia
http://www.themodernantiquarian.com/site/111
maen cetty
http://www.themodernantiquarian.com/site/4269
whittlestone
http://www.themodernantiquarian.com/site/6201
Maen Bredwan
http://www.themodernantiquarian.com/site/64
Longstone of Minchinhampton