I'm afraid I'm not much use as I haven't come across anything about them lately. I saw it mentioned in Dobson's 1931 'Archaeology of Somerset' but I can't remember what on earth it said now.

There's nothing in Leslie Grinsell's 'Prehistoric Bristol' about them.

there is this:
http://www.themodernantiquarian.com/site/2730
but that was Jim McNeill's first and last contribution to TMA - perhaps you could persuade one of the TMAeds to pass on his email address so you can give that a go?