Yes Colin, you're right about checking the 12V battery when the car is not in "ready mode".
To do so, you need to press the power button twice without pressing the brake pedal, the cars goes into "accessory mode".
In this condition, all the car is live, except ICE and HV part (Inverter, MG etc.), so the 12V battery will not being charged, indeed, you can see the red indication of the 12V battery not being charged on the dashboard.
Whereas, if you press the brake pedal and then the power button, the car goes into "ready mode", where the HV is connected to the inverter and thus also charges the 12V battery.