Proof of the pudding is in the Eating: I have rechecked the Carbon Monoxide levels and both carbs are running at 2.8 - 3.2%. After running up the motorway then killing the ignition and rolling to a stop both sparkplugs looked good, not to lean and not to rich. After 71 miles today of mixed riding the plugs look just the same as when killing the ignition on the motorway:
Peering down the porcelain both have a dark ring on the last 1/4 - 1/3 near the base. I am happy with the results and glad I took my time over Jan - March to find what the real issue was. Total cost to get here was £29 for a new second hand Hitachi carburetor body and £125 for the Gunson G4125 carbon monoxide detector which will be useful for the annual carb checkups. The rest was a lot of time and patience.
Thank you to all who have compiled their experience on this forum which has helped me greatly.
Edit on 23 Feb 2023: 300 miles later and the plugs look the same. The carbon monoxide levels remain stable at
Front Cylinder =3.2% (Pilot Screw 1/2 turn)
Rear Cylinder = 3.0% (Pilot Screw 3/8 turn)
Since addressing my carburetors I have 55 - 57 mpg, prior to this work it was 50 - 52 mpg. If I were to switch to 0% ethanol fuel then I should see an increase to about 60 mpg. Every 5% Ethanol decreases your efficiency by approximately 3 mpg.
Edit on 18 June 2023: After running a few months at 3% without issue and good plug colour I decided to try 4% Carbon Monoxide just out of curiosity. Adjusted the Pilot Screws using Gunson G4125:
Front Cylinder = 4% (Pilot Screw 9/16 turn)
Rear Cylinder = 4% (Pilot Screw 7/16 turn)
The bike seemed fine to ride but examination of the plugs after a 65 mile outing showed dark grey fouling on both cylinders. I also noticed a rich smell from the exhaust. So back we go to 3.0% Carbon Monoxide just like Gunson recommend...
Video regarding function of the air induction and reasoning behind the differences in main jet size: