if you have some thing on your property that could be unsafe or if touched could crumble ,cause a fall , however remote then i would advise putting a fence round it with a warning sign, eg open shaft, unsafe building, remember you are also liable for trespassers as well,
the other side of the coin that you should be made aware of is certain groups and societies will look at your old mine buildings /workings and may very well record by different means, what exactly is there, they may also inform the authorities that you have something on your land that deserves protection, this does not necessarily mean the building would be preserved or restored, but you would be finding yourself in the position if the building becomes a scheduled or unscheduled monument that you certainly on your own without permissions and funds would not be able to do anything, as would also be the case for groups and societies , what becomes a protected building/what ever, often is just left because the funds either private or public are not available to do any remedial work, as if often the case in ceredigion, you really need to think exactly what you want to achieve and take some unbiased advice /legal advice.