It came on again today when motor sailing but now at anchor its off again.
The SSB has an isolated power supply.
The laptop is powered from a Mastervolt inverter under the nav station.
It is connected to the SSB, GPS, AIS and a NMEA multiplexer...none of which have ground connections except the SSB, but the laptop is isolated from the DC circuit by its own power supply and the SSB also has an isolated power supply.
The leak is very small, barely noticeable on the lamp, so it must be some "device" that has both a 24V-ve and ground connection
I will try disconnecting things when the Masse- light comes on, but right now it's off.
Ile Puen, Baie de St Vincent, New Caledonia