Marcus Taylor did indeed film down there and produced an excellent DVD, which I think he will still sell you for something like ?12 ... a bargain.
I believe, but may be wrong, that you can't go to the GESM in the Summer season because of bats. I also believe that another cave in the same area has been extended quite a long way in the last 5 years and may now be 500m+ deep.
The final piece of info I know off the top of my head is that the GESM is 1098m deep and has a large (200m??) pitch about half way down.