Thanks to all, and special thanks to Roy and his huge help by PM, I am back on the road. It was solved by adding the new ground wire to the regulator casing and checking over every connector I could get my hands on. Bike actually runs a lot smoother than before (or is that my post-fix imagination ?). I now have 12.7V across the battery with ignition off (after a re-charge) and 13.85V at 1500rpm
GraeV11's post in dead battery ? was marked as the answer