This is maddening!
Stripped the dynamo and the springs and brushed indeed look fine. Fitted the new set of both just in case, and also cleaned up the brush runners as one of them was full of slight surface rust.
Put the dynamo back together, checked it motored ok (it did) and reinstalled in car.
Exactly the same symptoms, only now the light stays on once the increase in revs has set it off, even if you then back down to idle. It did exactly this the last time the dynamo was stripped by the specialist.
So, individually everything checks out to the level I can apply.
Auto electrician here we come, if I can find one who gets these...