Having previously had no luck getting a spark I decided to strip down the loom. Some if it was freshly wrapped but a lot of it was from the original bike loom so hadn't seen the light of day for 18 years.
The picture shows a slight problem in that there is an air gap in the  12V supply to the CDI unit (not the reduced voltage grey wire this one's brown/white 12V)
Time to clean it up and splice it back together.