We docked without mishap in a blustery Isafjorder and Muki the harbour master immediately made half a dozen phone calls to find a marine electrician for us. One hour later Hannis arrived and set about diagnosing why both hour 24 volt and our 12 volt regulator had given up the ghost. Here is the answer to the riddle that took 3 hours of his time to find and partially fix.
The 24 volt alternator ignition cable is routed from the 12 volt alternator to the switch panel and thence to the 24 volt relay under the chart table and thence to the alternator. The 12 volt alternator is defunct so we ran a bypass wire from the ignition cable at the 12v to the relay. So now we have 24 volts and we can manage without 12v until Reykjavik where we can repair or replace!
Thank you Hannis.