Oh all right then Steve
progress to date:
Rebuilt LH side, checked carbs (again), taking care to get cable and stop adjustments as close to identical as possible. Gave it a boot and off it went. It's still not running well, but fiddling with mixture and throttle stop improved it a bit, but still not as it needs to be. The centre stand on this bike is not a standard item and it lifts the rear wheel about 2" off the floor, so I ran it in gear up to about 40MPH
during which there was an intermittent misfire. It starts when on full advance, but not when retarded, and the carbs need tickling to get it going again.
Prior to the above, I had swapped the carb from the LH to the RH (no carb on left cyl)
And it ran.
All in all, so far there is nothing absolutely positive to conclude, so a bit more adjustment is required.
Now, look here, Steve, I happen to love NZ, and you sound a knowledgeable bloke, so I reckon I should come there and you should come and fix this piggin thing