Sadly the bulk of Pandora (mineral rights and all) are predominantly owned by a farmer who was poorly advised by the FUW in that he'd be liable if there was an incident in the mine. He rather aggressively withdrew access and has been very unwilling to discuss re-opening access. His primary concern comes from almost dropping a tractor down a shaft which had been capped with railway sleepers and grassed over so he was unaware of it! Over the years a few of these shaft tops have rotten away and fallen in; clearly he's concerned about people wandering off the footpath and down one of the holes.
Theoretically you could access via the lower levels and stay within what was the FC boundaries (now NRW), but for that you'd need permission from NRW as there is a by-law prohibiting access (so it's a criminal rather than just a civil issue). Lovely mine, but politically best to stay away.
As for the Klondike and other mines within the gorge upstream of the gated mine; they are all pretty small (beware of the one right of the stream looking up, shortly after the hole where the water drops into a short but of workings) as there's a flooded shaft almost immediately within the entrance. Far enough in so you don't spot it, but close enough to the entrance where your eyes are not used to the dark and you can fall in. It's been the scene of various near misses over the years.
The gated mine is a very short trial. The land owner had an "asset management consultant" take a look at his land and noticed that many outdoor centres were using the mine with groups. They suggested he gate it and ask for a pound per person to enter so he could put his feet up and retire on the profits. The gate must have cost him a small fortune, and no centre was willing to part with cash for such a short insignificant trip, not to mention the shift in liability to the land owner, so I assume the land owner had a few choice words for his consultant!
Again discussions on re-opening the mine have fallen on deaf ears...