I've obviously given all that a lot of thought, fear not.
Everybody but one who's broken in recently has used small cheap cordless angle grinders and/or bolt croppers to cut the chain and/or padlock.
You can't do that any more. When you see it you'll agree. The cellular-linked 24 hour PIR CCTV system going on it shortly won't buy anyone anywhere near enough time to get through that much steel before the cops rock up.
Yes people could loose the key in there, that would be rather stupid but it could happen. Many mines use a key to get in and out though, that's life. Don't' loose the key and make sure people know where you are.
However, to mitigate that risk the new door going on the smoke flue shortly allows exit without a key (like a night-latch).
Yes I'm intending on supplying a key to the frequent visiting groups (Thursday nighters, ucet'ers etc). I'm not quite ready yet but yes that's the plan.
No a Derbyshire key system isn't suitable in this case. It was suggested to the land owner some years ago and he wasn't satisfied.
Besides, I intend to make it easier, quicker and cheaper to borrow a key than it is to buy a 16mm hex off eBay.
Beneath my steely exterior beats the heart of a dashing hero