Jess, thanks for the reply. You're right, NC isn't obvious at first but it makes sense when you start thinking about the safety aspect of it.

I didn't get the multi meter out to test the switch itself in the end. I did think about it, but instead I tried running a wire between the two estop wires (not the switch) to close the connection (which would be like a functioning estop button) and it did nothing to reset the estop (through the diagnostics panel on mach). So I assumed it wasn't the button... The mystery goes on... Might be being pessimistic, but I think there might be a problem with either the controller card or the breakoutboard... neither of which I have seen because they came with the internals of the machine!