I finally gt to the root of the problem. I ended up putting new brushes and a solenoid onto an 034 starter after finding this inside the SA-made 22S part:
Yep. The plastic housing had cracked, meaning it would crank for a short while before winding the reduction gears out of mesh (so failing to start and just whirring) before snapping back with a thump when the key was released.
Explains all the symptoms.
1985 Oettinger 3.2 Caravelle RHD syncro twin slider. SA Microbus bumpers, duplex winch system, ARC 7X15 period alloys