Disagree your last in Colin
Mineral ownership whether separate from the land ownership or included would not confer the right to cross another persons land to get to yours, land, mine or whatever, without their authority.
In the real world there has been a lot of issues with peoples properties in this way. People have bought home in the countryside with a track leading to them and then discovered that the right of way is pedestrian access only, a solicitor is paid to search for all this.
A while ago someone bought the Lordship of a Manor which included a strip of land alongside a road which residents had to cross to get to their homes.
Basically, he was carpetbagging and wanted to charge exorbitant rents for people to get their cars into their own drives. The local authority sorted it with a compulsory purchase, job done.
Its complicated business. A piece of land can have one landowner, another mineral owner, another having grazing rights, and another having the sporting rights, never mind 'wayleave' etc.
To answer the question directly, that part of the mine that is under another's land is not your mine.
Rampgill is mentioned, there is a boundary gate underground that separates what was LLC and later VM from Beaumont ownership.
My avatar is a poor likeness.