Dave, thank you. I have now reassembled the starter motor and bolted it back into the "donor" car. On switch on ... nothing! Well, not quite nothing. There was a click, indicating that the solenoid was activating, but either not feeding current through to the motor or there was something else wrong with the motor.
I dug out my test light bulb on a bit of wire ... and discovered that there was a dry solder joint at the bulb. After resoldering and testing the bulb, I attached the test wire to the solenoid "output" terminal and turned the ignition switch. This time the bulb lit up ... and the starter turned. And it has worked every time since.
So ... all I really did was to dismantle, clean, lubricate where appropriate and reassemble. I didn't remove the brushes assembly as it all looked OK. My hunch is that the real issue was the solenoid armature sticking as I was never aware of a click when I had the original fail-to-start scenario.
At least I am now much more confident in removing and servicing the starter.
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