Hi George A, thank you for your reply. Things have moved on quite a bit since my post. I was told by others that the rectifier needed replacing and I did that, replacing it with a modern equivalent. I also replaced a capacitor that had taken a hit (one that's also part of the detection circuit).
My SOM has a new issue now. In manual mode the engine fires up as it should and the generator produces power. In automatic mode, when a lightbulb is switched on, the system detects the load as it should and goes into its start up routine. The solenoid on the engine operates and the engine starts and cranks up to full speed - its at that point of the engine hitting full power, when the main relay should operate and switch the engine from its starting mode to producing full power that there's still a fault manifesting itself. The main (multi contact) relay bounces and never fully settles, causing sparks across its contacts. Clearly (in my mind) there's still a damaged component somewhere, that's not allowing the system to settle into fully operational automatic mode.
I have decided that I don't know enough to take this issue any further myself and I also don't want to damage any other components. Given we are in lockdown, its not possible to get anyone to come and take a look, so for now, I will continue to run the engine in manual mode.
If you have any thoughts of what other compolents I could test or just simply replace, that would be very helpful.