Sorted, for now.
Tried using the battery negative and the female positive bullet connector on the front brake like switch at first, but that didn't work. Was going to tap into the rear license plate light using a splice but got half way to dismantling things when I realised I could try using the ground male bullet connector from the horn (see above) that was left exposed on removing the O.E.M heated grips combined with the female bullet connector on the front brake light switch (always had a feeling it was one complete circuit).
The result was power when the ignition was turned on (engine not running) and no power when the ignition was turned off.
Battery at 13.8 something volts when running, and 12. something when not. Perfectly healthy for now. All circuits work fine (forgot to try horn).
Cut off and added a new connector on the horn side as the old one had been exposed to road crud for a while, and then wrapped all the wire I had to remove the rubber cover from to add a little bit of protection against the weather. Heat shrink would have been better, but what works works. I really wasn't going for a professional install here.

Routed under the R/H side fairing (literally just fed it through) and tided up the bottom end with cable ties to avoid it dangling.
There's an awful lot of excess wire that connects to the TomTom charger dock itself, and for now I've just cable tied it to existing wiring because I'm still waiting for my Migsel mount so I can a "final install" and test run.
I may cut it down when I get the Migsel mount delivered because then I'll know how much length I actually need. Expose the wire and then add a bullet connector to either side (positive / negative) to complete the circuit and wrap it in some tape.
edit: I am going to have to think of something to do with the exposed connector that plugs into the TomTom dock connector, though, because I don't plan to leave the dock on the bike (pre-planned days of using the sat-nav). May just keep unwrapping and rapping it with the same electrical tape until it needs replacing and so on and so forth. A rubber cap that would plug into it to protect it would be nice.
