The low voltage light problem was a wire that had vibrated off a connecter spade and has now been put back on correctly, we think.
The switch is a double rocker type. One half battery and the other the generator/alternator?
I agree that the power to the radio and headset intercom should of gone dead when the battery and alternator switch is turned off but whilst the engine was still running at 2100 rpm, did not do this and was allowing me to use the radio for ATC and I was able to still talk to the guy sat next to me through my headset.
It was only when I stopped the engine and tried switching off of the double master switch for a second time did it cut all power to all electric systems ???