I am not sure about the exact situation.
If you really want to know, post the question on Miles's site for attention of Simon H.
Simon Hughes posts on there and he will know.
Failing that, I will ask him this weekend, as I stay there every Saturday night.
Thinking about it, it would be an interesting target for one of my projects, depending on the extent of the adit workings and possible backs. The fall at the engine shaft could easily be spiled through, and there is the flooded understope right outside to dump into. I would not be inclined to descend the shaft at the back as its almost certainly blocked a short distance down, and probably with the most revolting material. I once had a bad experience in the Lake District this way. To your list of mine gasses, CO2, H2S etc you can add 'dead sheep gas'.
For those who have not been following the Talybont project, my friends and I have driven a whopping 120 feet through collapsed ground at almost full walking height. We have also now started a new project, location 'secret' while work ongoing.
We really have it to a fine art.
My avatar is a poor likeness.