I measured the contact pin width and went off to Jaycar to get the right contact plugs, some wire, and a switch. I also got a bit of heat-shrink tube to make things a little more tidy and protect the wire a bit. This is the bit that inserts between the thermo switch contacts and the wiring that powers the fan, and the wire that will head up to a switch up near the steering head stem.
This is the new piece in place. The advantage of doing it this way is that there is no cutting of the stock wiring, and no soldering (which I am particularly useless at doing). If there is ever a problem with the switch I will simply unplug my extensions and plug things back in their original configuration.
I have installed the little switch up in front of the tanl on the steering head stem near the engine map switch that I recently moved and mounted to the steering lock.
Now, whilst the switch is "off" the fan if off until the thermo switch kicks in if the bike gets hot enough. I don't have to worry about boiling the bike because of forgetting to flip the switch. But at any time I can flip the switch "on" and the fan spins up. So when I know I'm going to be doing some slow pace riding where the bike will get hot I can just flip the switch and allow the bike to run longer before it gets too hot. I expect this to be particularly useful in the summer months.