Well, this is a story with a happy ending! Eventually traced the issue to one of the pick up coils, the lower one to be exact. These were removed and then reinstalled as part of the rebuild. The insulation on the lower pickup was a little worn out and was grounding to the engine. This meant no spark on either plug.
To check:
-Disconnect 4-pin white plug under seat.
-Check each of the 4 pins to see if any of these ground to the frame
-Re-insulate as needed
Apparently this is a common thing, particularly with the lower coil which is more exposed to getting a bit of fuel etc on it from time to time.
I was a so happy when I found this as I'd starting searching for a new PGM!
Thanks to ****** for the advice here. You're a champion!