Like this original stone in the foreground:
http://www.themodernantiquarian.com/post/83149/devils_quoits.html
I could buy that explanation for the flatter stones, but not for the larger stones. And I come back to the fact that the henge and ditches look extremely quoit-like:
http://www.themodernantiquarian.com/post/15207/devils_quoits.html