The main jet size won't have any effect below 3/4 throttle.
Make sure the choke slides are pulled fully up by the choke cable,or just remove the choke slides and plug the hole in the carb top.If you plug the hole,make sure whatever you plug it with can't come loose and damage the engine.
If it 8-strokes at full throttle with 200 mains,try 180.2 sizes less than the smallest jet that causes 8-stroking is usually about right;it's a lot easier and quicker than trying to read plugs.
Ok lads, progress so far. Totally disassembled my carbs, checked the main jet was cleared, removed the chokes and choke cables (do we really need a choke in Australia's climate?) checked the float levels, reassembled the carbs with the clip at the top ring and needles dropped as low as possible.
Checked and reset valve clearance front and back - inlet was tight, possibly incorrectly set by me to the ex setting?
Changed the fuel to BP 98 (had Shell 98 - now a no-no, apparently).
Tuned the carbs for tick over and 1/8th throttle - is now approx 2 and 1/4 turns - not as rich as it was. Seems to idle quite happily now.
Inserted new, hotter plugs NKG B6ES - went for a short run and she revved and pulled as hard as I was prepared to go considering I'm running her in.
Running rich appears to be my perpetual issue, so if this hasn't worked, it's the 180 mains and back to a middle clip on the needle?
I don't want to introduce too many variables into the equation or I'll get lost chasing my tail.
Overall, feeling less frustrated.