I finally bought a new 3AJ CDI unit for £200 and still no spark!
I double checked the resistance in the source and pickup coils and they seem to be OK, although the source coil is slightly high (144 instead of 112 - 132 Ohms as specified in the 3AJ service manual, my spare 1VJ Source coil also reads 140). I also checked the AC voltage output from the source coil and it showed about 47 volts.
I've just discovered that there are two relays that might be preventing a spark. #1 The Neutral Relay and #2 Starting Circuit cut-off relay. Thinking I had better check these, I swapped the relays from my 1VJ wiring loom and heard a click from #2 when the ignition was switched on and off. This seemed to be an improvement on the silent relays.
For some reason the 20 Amp fuse blew and I have now I have lost the green neutral light and the start button doesn't work anymore!
Lights and indicators still work...
I have searched for information about the relays and have read that some people bypass them.
Has anyone here done this on a UK 3AJ and can advise what colour wires to connect together?
The Neutral Relay has: blue/white, blue/yellow, brown and light blue
The Starting Circuit cut-off relay has: black, red/white, brown and light blue
I'm so glad that after all these years, thanks to @Mezo, I finally found the source coil resistance for a 3AJ and I can ignore the nagging doubt from my Clymer manual. I have 144 ohms, so my stator looks to be ok.