One thing that struck me different about mine water is it's chemical loading, resulting in direct reaction with acid and metal salt substitution reactions. I remember a post about Wheal Jane, which would have some spectacularly huge stopes, but would be anoxic and in those conditions, your bottle/kit would be pretty much soluble.
I'm interested in this topic, it's probably a bit beyond me.
On the helicopter front though, my "have to do" concerns autogyros. You can pick them up for pretty cheap and in the old days, they came with a "these things can kill you" warning and an instructional cine-tape.
Nanny knows best and armed with a suitable amount of balls and common sense, anything is possible. My concern would be (for cornwall) that fine mud is a constant companion, as are small tunnels and this would lead to a blackout and feel your way out. Where bends is concerned, this must take a huge amount of skill and elephant balls.
I will stick to autogyros.