I'm making progress
Removed the dynamo, cleaned the commutator and checked the brushes, then refitted it. Ran the engine and checked voltages, they were OK but still not charging. Connected F to D on the control box, that bypasses the regulator. Then the dynamo produced the expected voltages and charging current. Couldn't sort the Control Box so bit the bullet and ordered a new one.
Found some helpful articles on the Morris Minor Owners Club forum.
