Despite the humidity I have managed to solve my wiring problem, at last!
Other than the switch that is . . .
. . . the switch was straight forward to swap, even though the Jaycar salesman had extreme difficulty pulling the wires off the contact blades. No charge, and I was on my way again. Once the switch was replaced I hooked everything up and it worked (almost)!? Everything right, except the small lights came on when they felt like it, naughty little buggers, they even persisted and stayed on with the ignition off.
I resorted to going right back to basics with the fuse block, after finding nothing even remotely suspicious with the wiring at the switch panel. Days, hours, and much perspiration wasted. I could not fault any of my wiring, or connections.
Mulling the situation over last night, I couldn't sleep, so up at 2am and out to the shed. Much cooler and not a drop of perspiration; I did more in half an hour than I'd done in hours previously. I pulled every fuse out and worked my way, wire by wire looking for anomalies. The separate sections within the
cheapie re-wiring kit fuse block were live when they shouldn't have been - this was rectified by separating the wavy contact pieces that had been cut to prevent power passing to the section alongside (on both sides of the fuse block

) but were still managing to make contact! A small electrical screwdriver pushed between the sections and the wavy connecting strip was pushed back in either direction, leaving a much larger gap than previously. Result was a resounding success, and now awaits slotting the switch box into place.
Along the way I made up a better wiring diagram than the simple one supplied with the kit and which I had colour coded to assist my tracing of wires. After much referencing this was becoming decidedly grubby, and as there was no wiring diagram at all for the switch panel I made my own, so I went from this:
To this:
And after a busy morning at the Men's Shed made up a new wiring diagram for the fuse block too:
A wearying experience but feeling much, much better now.