Thanks Mark but I think we'll be okay for now. Desperate to avoid a key again here.
I have been up today and the new combination lock has died an untimely death, despite not being in place for more than 2 weeks. I had my grinder with me just in case so I have chopped it off. Until we've had time to have a quick think about the replacement, there is no combination required for access. Please send a notification email in advance to the owner before visiting, details on the DCA website.
Pete Knight
DCA Projects
P.S. if someone has a short M8 bolt (13mm head) and a nut they want to donate then please feel free to pass it on or even better, fit it to the old padlock hole yourself to make it secure again. I had hoped to avoid the need for cavers to take anything up there, like a key or spanners, hence the combi lock.