Quick update, just in case it's useful to others.
Seems this 'Rectifier problem' was actually due to the main 12v power feed to it being chaffed through so that there was a short to earth. The voltage drop was somewhat intermittent and clearly only dropped to 12v on the gauge when clean contact was being made. When I stopped on an uphill section waiting at traffic lights, I got back up to 14v. Feed wire repaired and now shielded properly with wrap, so hopefully job done, but I can't fire it up right now as the exhaust is hanging off....
... But I have been able to remove the sheared stud from the exhaust port. Soaked a little WD40 on the threads and left it for 30 mins. The heated stud with blowtorch and used a proper Stud-Extractor socket (8mm) to carefully undo and withdraw the broken part. Came out really easily. 4x new studs and nuts coming (about £5 a pair for stud+nut), so I have some more spares. So the 8mm stud-extractor is now another must have in the on-board tool kit then! I await delivery...
