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