Off the top of my head, I think the Vol 4, Penzance and Mounts Bay is the Cornwall's Mines and Miners book that you want by Hamilton Jenkin.
There may be one in your local library.
I remember someone mentioning that the adit was blocked by the creation of a road, but there was a footway shaft open. If that someone is reading this, get in contact and my chum and I will drop it and have a poke around and see what we can find.