Update- last driven 23rd May, driven today. Front camera had kept the time/ date and started up okay. Rear camera had lost the time/ date but started up okay. (I've swapped them around since last update.)
Phil, I've got push button start. So the car goes from no power to accessories/ ignition, straight to engine cranking (with foot on brake, pressing start button.) I deliberately put an "on delay" circuit in the front camera's wiring- it's the one powered off the interior lighting circuit. The idea being to provide a delay long enough for the Mobius to "sort itself out" if it does happen to lose power when cranking. It's got me thinking that something is happening after the car sits for an extended period (ie. capacitor very low/no charge) and that is causing the failures. (Remembering it works fine if driven every 5 day or so.) Perhaps I need to extend the delay circuit time or completely remove it? The rear camera (with the lost date/ weaker capacitor) is also the one that was "playing up" when mounted in the front with the delay circuit.
(The car is 12mths old and cranks over with plenty of power, so at this stage I'm not pursuing the cranking battery voltage being the problem. I will keep a closer eye on what the interior lights do when starting the engine each time. The vehicle does have a battery monitoring system that could be doing strange things after the vehicle sits for a week+ in order to maximise battery charging.)