In reply to your request for more information.
When we first "discovered" the tunnels as children the army where in residence at Welbeck College and there presence was security enough. Since they have left i am told the abbey and it's attached buildings have been split into some sort of millionaires apartments. The high level of security now in place at Welbeck (cctv etc) i assume is to do with this. There are still areas that can be accessed but you would need local knowledge to understand any directions i could give you.