Brilliant, thanks Paul, that was just what I needed. I soldered my connections at the switch to make sure there were no frayed strands causing shorts and connected as you advised and all is working perfectly.
In case anyone happens across this thread with the same problem, the connections were as follows:
Switch ---> ControlBlock GPIO
( + ) ---> LED +
( - ) ---> LED -
NO1 ---> Switch +
C1 ---> Switch -
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first