... and so we move to the final chapter in this epic saga of lost power and endless head scratching.
The 'split' lead (original HD, on the bike from new) from BATT. NEG to frame / engine has been replaced by 2 new leads fulfilling the same purpose. All is now as it should be

... bike starts / runs just fine.
The lead which was removed has been tested and has no measurable resistance ... it just doesn't work !!!
How can a piece of wire just not work ? I don't really know, perhaps there are changes to its actual molecular/atomic structure over time ... now that IS deep :s The only comment I've really heard coming from a couple of auto electricians is that ... "earth leads can do funny things" ... indeed.
So there you have it. You can disassemble/clean/reassemble contacts 'til you are blue in the face. Bypass leads and components and measure resistances but this lead just simply had to be replaced, even though there is apparently nothing wrong with it.
It's done, I'm on the road. A million thanks to
everyone who contributed and if this story helps anyone in the future it will all have been worth it.
cheers
RR