Jonathan, your first port of call should be the ex Wheal Jane guys (that are still around), Wheal Jane did quite a bit of work on the adit between their site and the Nangiles portal due to the connections with their drainage, in the 80's I think. Also would highly recommend Buckleys 'Great County Adit' book, out of print but there are still a few copies knocking around! A few chaps on here have done bits of it as well, they'll know something of the state of it now. Worth getting in touch with Agricola, one of the members of this site