Thanks both, Quite pleased with the all round white confirmation. Any other comments welcome. Saves the grief of devising a hauling system that keeps a tricolour in the correct orientation, and the thing would work better as a general purpose backup lamp.
Regarding systems for mounting, I was going to use the system shown in Bob Harlands article under the cruising/equipment section – search on the homepage for masthead lights and you will find it. I was going to modify, as I assume he has, to fix the magnet plate on top of a tube. I think the various collars on the tube a) stop the tube battering the mast and b) act as security for hitching the device to a halhard like a burgee. Tube to be long enough to project lamp above mast head without flopping about.
Those are my ideas anyway.