Just a thought looking at Richards diagram…if you have difficulty testing the three diodes, why not disconnect the hot wire from the diode triple point to the ignition light and pop a suitable diode in series with it to see if the light goes out when reconnected in that state.
This is the trouble with my understanding of electrickery. I know the basics, but no idea about connecting a diode in series. I don't think I have one in my shed
