Ok, I got the audio working.
So, what I've done is to keep using the Honda "premium audio" amplifier.
I cut the leads in the Sony ISO harness, so instead of connecting to the little black plug that goes into the sony unit (which are amplified), I've terminated them in 4x RCA/phono connectors, which then go to the preamp front L/R and rear L/R connections. +ve connection to the RCA inner, -ve connection to the RCA outer.
I've also tapped into the subwoofer connection on the Honda wiring harness (AUDIO UNIT CONNECTOR E 8P), and connected that to an RCA/phono plug, which connects to the back of the Sony unit.
Next, I've tapped into the standard Honda harness again, since the Connects2 CTSHO006.2 kit doesn't have the wire to turn the amplifier on.
This time I've taken the blue/white lead from the Sony ISO harness, and connected it to Lead 21 on the main Honda connector A, which is where the power amp gets it's signal to turn on...
And there we have it. It works. She chooches.
I still have some minor things to do:
At the moment, I've got the Sony USB connector coming into the glove box, which is a very minor inconvenience. I'd like to use the USB connector that's under the armrest in the centre console, but I've not found where that come out in the wiring loom, yet.