Thank you both for responding so quickly!
I should've included a couple more details which are quite vital. First being access/space. We'll need to bring tripods/lights/cameras/actors down there so scrambling in tight areas has to be avoided as much as possible. Ideally the mine would have some narrow shafts and some larger areas/caverns too. Secondly, flashlights obviously wern't a form of lighting in the 17th century so the actors will be using either lanterns with a candle inside or a handheld fire torch. I imagine the type of mine matters greatly when dealing with naked flames. Just something worth considering, its looking more likely we'll opt for the candle/lantern option due to the fumes given off of larger torches.
In response to Roy, it is a student project which is being made solely for our degree module, not for commercial distribution. Having said that, if the film is successful enough we may be entering into some small scale film festivals which have potential to bring back earnings, we'd be happy to draw up a deal to split any festival earnings should they arise? We have full public liability insurance and can share all relevant paperwork, and risk assessments. But Cwmystwyth sounds fantastic, would love to find out some more about the space!
In response to Alex, our team tried to contact WCMS about a month ago but haven't had anything back from them yet unfortunately.
Thank you for your help!