OK, the ST2 is installed and it works fine. I've not modified the indicator switch yet but the electronics are all working perfectly.
The good news is that the common indicator wire Pin 12 which is Brown on the bike side (Blue/Red on handlebar side) of the connector is a stable switched live as I suspected so as you're already tapping into that wire you can simply use it for the +ve to the ST2. No extra wiring is required other than a ground wire for the ST2 but that can be attached to any convenient earthed bolt.