Sorry I misled you!
I did swap the outer wires on my ammeter (there are 3 of them A(cutout) SH(dynamo) & B(battery). The cut out(A) is connected to the warning light I think, the fourth terminal goes to the other dynamo terminal I believe.
I haven't gone near the switches!
For the moment I think I will leave well alone & see how reliable the "charge" is
D

Location: Stratford upon Avon

Because the ammeter is built in to the switch panel it isn't easy to swap the ammeter over. Easiest course might be to revert to negative earth or live with the reversed meter.
I have a wiring diagram for the early coil engine cars which is much clearer than the one in the Companion. I can email you a jpeg of it if it would help. It shows the horn wiring correctly which the Companion one doesn't.

I don't fancy swapping back to negative at the moment, so will live with a reversed ammeter! More pressing is to persuade the dynamo/cut out to put out its amps more reliably...it all seems a bit hit & miss at the moment. I hope it is mostly years of inactivity. I might give the switch contacts a blast of contact cleaner (as used on modern electronics) & check that all the wires are tight in their various terminals/connections.
Thanks for the offer of a jpeg diagram, I will send you an email to make sure my address is right!
Happy Christmas!
David

Location: Stratford upon Avon

David, I admit to being a bit of a numpty when it comes to lekky stuff but I had the same issue when I purchase a 1930 saloon. It produced the same symptoms as yours because someone had put the battery in the wrong way round. As Jim says, it should be neg earth so if you swop the battery around it should sort it. You will probably need to 'flash' the dynamo field winding to re-polarise but I would wait and see what it does first. You may also need to check the polarity of the coil but I suspect that is as it should be.. All other things should function as normal. You should not have to swap connections on the ammeter

Location: Farnham