A public note of thanks to all for your help and insights on here.
So, turns out the starter motor is an inertia one, similar to the one in the link in John D's post above.
I managed to get the motor out (a tad fiddly, needed to jack the car and move the petrol supply) and it ta-da, was sticky as Clive suspected - maybe an oil residue?
Got it clean (not quite as clean as the one in John D's post above), but close - WD and then petrol.
Maybe it's possible to take the spring mechanism apart to clean the parts separately? Pretty strong springs though and not sure how they'd go back!
The car fired up first time, without issue.
So again, thanks all, it was indeed a learning process and I think my late-pa (who completely re-furbished the car) would have been proud that I avoided a garage.
The electrical thread is helpful too and all noted, so thanks for that also.