Yes Firewart, that was part of my problem. The OEM starter pre 1996 with the post type actuator connector is only available, so far as I could find, in Chinese copies or from Toyota at eye watering prices. ($1733 from Amayata) I bought a genuine Denso DSN1228 specified for 1996 onwards for £175 (which is what your '97 80 should have had on it). This has a spade connector for the actuation circuit so I altered the wiring to suit with a late model wire from a scrapper. At this point I had only copies of wiring diagrams which showed the starter being actuated direct from the ignition key in post 1996 80s with no starter relay. I now realise that all these diagrams are for US petrol based vehicles and having found the EWS supplement for post Aug 96 HDJ80s I am relieved to find that the diesel starter needs 24V on both circuits so I can just use the existing circuit from the LH battery fusible link. Problem now solved to my satisfaction!
No nearer though, in understanding what the little computer that controls the 12/24 changeover does; though, as it senses the alternator warning light, I assume one function is to protect the alternator from 24V if the relay sticks. Still waiting for a forum expert to tell me, or hazard a guess.