At this point, all the connections to the brushes and stator windings are exposed, so it is possible to check with a multimeter

Checking the stator windings and basic brush continuity is easy. To check the rotor fully, you really need to rotate the motor. It is possible to do that from the rear. There are teeth at the rear of the commutator which you can push with a small screwdriver. But next I outline how to remove the front gearbox: it is straight forward.

In my case, I found that one of the stator windings was open circuit. Eventually with a bright light and magnifying spectacles, I discovered the the lead just inside the casing was damaged and had clearly come in contact with the commutator. How did that happen? There should be some way to keep it clear.