Later Aero based cars, the ones with the 4.8L motor, have a digital voltmeter available to display in the small LCD in the bottom of the speedometer. I'm not sure exactly when this started, possibly the Aero S4.
I checked Tarka, following the process you descrived, and the results were spot on: 14.7v charging, 12.3v after a few hours sitting. Turning on the lights, heated screen and seat heater each drops the voltage by 0.1v, so with all on it showed 12.0v.
This suggests that the battery is good and being properly charged.
The switch pannel with the Warning lights and the strip of warning lights below are prone to internal corrosion if they get damp. On Tarka the row of warning lights failed one by one until nothing worked - but the button for the heated screen glowed very slightly and all the switches worked.
All replaced by MMC under warranty.
