The problem is...
I've created a new Generic Event in Jira (which is actively used in a workflow transition) but it is not being listed in the Events list now that I'm trying to create a Custom Listener to consume it. Older Generic Events are listed!
The same new Event is however listed in Script Runners - Built in scripts | Generate events form!?
I've tried clearing the Groovy and System/Jira caches without any luck.
I did reproduce the same behaviour in our DEV instance of Jira and the resolution I found there was to restart Jira - obviously I can't do this in our PROD instance?
Any assistance gratefully appreciated.
Garry