OK, I sort of gave up. After the £610 expenditure and the persistence of the fault I decided to call a halt to spending more on it as the fault was persistent despite everything.
I fitted a new set of plugs (because they were overdue for a change - so overdue I needed an Impact driver to get them out) and I decided to just live with the "resetting" as long as it did not cause the engine to die.
While changing the plugs I removed the earth from the Alternator (which was changed some while ago) scrubbed all the contact patches with a wire brush and smeared everything with copper-ease before re-tightening back up.
I also messed about with the drivers key-lock because the drivers door was out of synch with the "fob-opening". The fob would lock/unlock all doors except the drivers door itself, but using the physical key locked/unlocked all doors.
Now, during all my messing about I managed to fire a "mislock" alarm after which the driver doorlock came back into synch with keyfob opening/closing requests and all locking with the fob is now correct.
Spooky but true....... Since this re-synch between the fob and key locking modes the "fault" that has been plaguing me for so long has ceased.
No more Beeps from the console, no "low-fuel" or "Over-Temp" lights, no sudden stop/restart of A/C if its running.
Oh, and MPG is currently up at 11.6MPG and climbing.
Now, I am not claiming a "FIX" has been performed, but I wondered if anyone had an insight into why the fault could possibly disappear in this way.
I'm still going with dodgy earths as the most likely culprit.
A big thank you to everyone who has assisted with advice & guidance.
