Forgot to update the thread,
I picked up an oem Mikuni on ebay and fitted it, started first kick. Needed a bit of fettling to set the idle and stuff but been a couple runs on it and it seems to run great.
I gave up on the battery eliminator/capacitor thing. I think it might work to keep the lights illuminated on a bike with an alternator (maybe?) but it wasn't working on this and the ammeter was all over the place. I picked up a battery and the lights and horn work as they should and from what I can tell, it's charging properly.
Oh nice! Proper cool bike.
Dyno tech ecosse at Hillington, that guy looks after a few of my customers old enfields with Amal carbs, so he is very 'up' on them, often you'll get the right casing and slider size/spring, but the jetting will be basically random compared to what you have ordered or require. He likely has a shelf full of bits for them ;)