As promised, herewith the photographs, and a simplified drawing. The first photo is just of the outside of the panel, but shows where I mounted the switch, the Dymo label is a bit naff and I’ll change it sometime when I’ve nothing better to do.
The next photo shows the reverse side of the mains breaker switch where each of the cables for the various mains electric circuits it provides for. Notice that each of the cable ends is held in place by a small grub screw that tightens onto the bared wires. There are at least three rows of connections, with each row handling a specific service, namely negative, positive and earth wires. One of the first things I did was to identify the cable lead between the mains breaker and the battery charger. On my boat that had a grey plastic outer sheath, and in the vicinity of the mains breaker there was only one cable lead of that colour. Disconnecting one of the wires proved that this was indeed the cable whose wires I needed to look at as that disconnection had stopped the charger from working, but had not interfered with any of the other services.
Fitting the switch meant first cutting a rectangular hole into the face of the mains panel. This was done by marking a rectangular shape of the right size on the surface of that panel, drilling several small holes that I could link together, and finishing off with a flat file. With the switch in place it was a simple matter to fit spade connectors to some new wires that would connect from the breaker to the new switch, photo 3. Spade connectors were also fitted to the wires to the charger and were connected as shown in my diagram in the fourth picture which I hope is easy enough to follow.