I've been there very briefly and wondered whether the earthworks I saw were the original Iron Age ones.. Anyway I would refer you to the English Heritage record, but that's not yet up on the MAGIC website. But maybe you could contact the Bristol City museum? 0117 922 3571 I'm sure someone there would be able to help.
[edit 6/07 - the weblog here
http://www.themodernantiquarian.com/post/55887/weblog/
gives some mythological explanation for the earthworks, not quite historically accurate, but maybe interesting)