Well said Nick,
However, I am becoming more agreeable to the 14 mm bolt/ nut idea, which seems to be in use on various gates around the Pennines.
I think anyone who is contemplating going underground can carry one of these with them, or an adjustable wrench.
This simply keeps out the inexperienced, and at least some thought must be made before going underground.
I guess we have all seen parties in Smallcleugh, usually I have observed, with minimum lighting and even in one case I recall without helmets, hopefully the bolt idea might deter these.
The main concern is that people could simply get lost in both mines, and where would start looking?