the mine owner and the mineral rights owner are two different entities, unless the are the same ( so to speak ) - ie the mineral owner is NOT responsible for the mine unless they were the mining operator as well.
Combe Down mine owners were long defunct and as such, the rights defaulted to the local authorities ie Bath & NE Somerset Council ( BaNES ). Legal advice was that since the workings were a continuous series with multiple entries, the property owners could NOT be held to be the owners.
The workings were nominally transferred to the property owners once stabilised, so they are nominal "owners" responsible for preventing access to a solid block of concrete - not too difficult. BaNES are still the owners in law, for the remaining accessible sections, regardless of who owns the freehold at ground level.
I do know for a certain fact, that the mineral owners are NOT responsible for the tips. The mine owners, or their successors, are responsible for making them stable and preventing pollution from run-offs, but that's all.
much of this was clarified following the Aberfan disaster, which was caused in large part by a lack of coherent guidance on the matter in law. The Davies Enquiry established without doubt that the NCB, as they then were, had complied with the law - such as it then was - but this meant in practice that they had no overall policy and no requirement existed in law for there to be one.
The Cornish tips belong, by default, to the land-owners who may dispose of them as they see fit, in any legal fashion. Many of the tips round Brae were sold, washed for whatever could be extracted, and sold on as MoT Type 1 when the Camborne Bypass was built.
plus ca change, plus c'est le meme chose