We never could find anything wrong with the dynamo, but the battery clearly wasn't charging. Perhaps it was the regulator (bad news if so!). But to give the dynamo its best shot, we took it to be looked over by a man in Hednesford, and collected it this morning, duly looked at and serviced, and assured that it was in full working order, and with a wiring diagram (given that thde one we were working to previously might have been wrong. So Jim connected it up and... still nothing getting to the battery.
Lots of conversations ensued between John in Hednesford, John here at Stretton who was generously applying his electrical engineering knowledge, and Jim, none of which in all honesty I really understand, but the upshot was, after trying almost everything else, to disconnect the ammeter (that's not it in the picture, that's the oil pressure guage. If you want to see a picture of the ammeter, scroll back a few days). Magic! A quick rev of the engine, and power was going to the battery, and at a good rate too. I can now extravagantly charge my phone without guilt!