As I understand it, all land & minerals in the UK belongs to the Crown UNLESS you can prove title otherwise.
As I understand it, minerals are held to be in the same ownership as the surface unless specifically seperated.
"With regard to the Crown Estate's attitude to recreational mine explorers, I'd concur that they are not going to be welcoming; and that they wouldn't willingly enter into access agreements with groups." Depends upon the attitude of he local mineral agent; experience suggests you are, unfortuntalet, all too right though.
"I'd also worry that they have the financial ability to secure adits rather better than most." Very, very true. The Crown Estate turns a profit of £90 to £100 million p.a. so if local agents spend a few thousand her and there on grilles (they tend to grill rather than agte) and fences then it doesn't exactly dent the overall income.
"Take the example of a gated adit. The gate prevents entry into the mine therefore it is protecting the mineral owners' property!"
And, more pertiently in this modern age, it is proptecting the liability of the mineral owner and/or landowner.
"If the mine adit is not gated, who is reponsible for it then?"
Usually the mineral owner. Or, more prosaically in this modern age, the wealthiest interest associated with the land in question - that's who a claimant's solicitor will target!
"At what point does the "ownership" transfer from land owner to mine owner!" As I understand it, the mineral owner remains liable - but an opportunist solicitor will likely sue any and all of the interests that might be financially worth pursuing!