Baza's link to the Delaney Park neolithic site has another interesting link - Clifton Antiquarian Society -

http://www.cliftonantiquarian.co.uk/CACrhossili.htm.

George Nash (a hero of mine) seems to be involved, and the society have also done some exploratory work on other sites; Rhossili above, Caer dyui (possible cupmarks) and Arthur's Stone.

p.s. Clifton of course is in Bristol....