Jezz mate welcome to "club CB650 charging problems" !
Many on here been battling this one, I'm hoping that one of us will find the secret to this issue.
I have replaced my stator with a borrowed one which looked a promising start but again no change, replaced the reg/rect with a borrowed unit but no change, have done all the Clymer tests and no problem found.
Interesting you say you replaced the stator and got it charging for a short period!
It is the one thing I have yet to replace, hmm my wrecker has a couple of spares, might borrow one of those too.
If you have been following "Kit's" tale of woe in her HOT START ISSUE thread you will know she has gone into even more depth than me.
I do know mine keeps a charge much longer if I ride with the quartz halogen light off when riding but I like to be seen and for me this is an unacceptable safety issue.
What do you experience if you ride without your headlight on?
Does your bike start OK but after riding it wont restart? or is very sluggish to turn over?
We had theorised that the starter motor could be at fault, then blamed the pulse generators, at this point we just don't know???