Last night i studied the documentation, the one worth a read is the archaeological report near the bottom of the list.
They miss the open adit completely and describe the shaft subsidence as a "quarry scoop". The presence of a mine shaft or shafts poses H and S issues for contractors working on site. As does the possibility of surface workings on the lode back. The remains of a building they dont even associate with mining.
It has been many years since I explored the adit. I have a vague memory of it being a bit awkward to get to, being in good condition, and leading to small workings in the roof with a collapse, probably from the filled in shaft.
Anybody been in there recently?
My avatar is a poor likeness.