A bit of an update...
Well, the next day the screen was still black when I selected Reverse.
The day after that there was a blue coloured area about 15% of the screen, down one side, when I selected Reverse.
The day after that, it was working again - perfectly.
I think, with it not having been driven for a while, it was some moisture or condensation?
Driving to work I have kept the heater hot and the air flow... flowing.
Long may it continue.