IGNITION SWITCH

[bcmc33]

Since the alternator feeds straight back to the battery circuit, your symptoms would be produced by a faulty ignition switch which doesn't actually disconnect the wire to the ignition coil.

That was always my first thought, Bill, and I think now confirmed by me tracing some of the circuits that are far from being standard.
I shall order a new switch on Monday, and wait to see if the issue returns.