I'm not able to loosen the screw that holds the end cover and don't want to tempt fate by trying more brutal methods. Could I be able to work out which way around they go by looking at the ammeter with the bike running?
No you can't...Loosen/remove the screw that retains the brush cover band (the black steel band that goes round the dynamo...) and the band can be removed without taking off the Bakelite end cover...
You'll then be able to see how it's connected...Under the brush cover band you will find a cork or thick paper gasket which may or may not be attached to the band...That will have to be removed as well...Ian