It turned out that it was the external charge regulator that was the problem. This was the top end Balmar MC-624 Maxcharge multistage regulator that had mysteriously died after just 6 years of service. Whole think is sealed in epoxy so cannot be serviced or repaired. Expensive bit of critical kit!

Last autumn a boatyard wanted to move our boat but couldn 't work out how to so they jump started it by putting a screwdriver across the ignition relay. Now, at the start of the season, we discover both our 12 volt and our 24 volt alternators are dead. Of course I can 't prove it but my guess is that the diodes have been blown when they ran the engine without turning on the ignition.

Not a great start to the new season. Anyway both are on their way to be tested and repaired so we will see what the official verdict is.