I've checked all the electrical side, and there is nothing wrong with my horn - other than it not working! I've drilled out the rivets, cleaned the points, and find that when loosely assembed it works after a fashion. The diaphragm is rigidly fixed to the shaft. It looks as though it is rusted onto the shaft. I suspect that it should be loose on that shaft. Can anyone confirm?
New horn!
I fixed it! The diaphragm was correctly fitted on the shaft. The issue was adjustment of the screw - which was a long way off. Key thing was if it makes a single 'pop' you wind the screw to open the points earlier. And if no noise at all you wind the opposite way.