So the carb rebuild had very little effect in the end. Setting the low end needle for good transition on the ground still leaves the low end too rich. It was still worth doing for educational purposes however and did reveal that even with 3 levels of filtering the carb fuel screen can still clog up.
One piece of info which my be useful is that I ordered the K20 WAT kit as well and that has both a large and small needle. I haven't tried it yet but it looks to be the right size for my OS 22.
...and it was 2 quid cheaper than the K10 WAT so I've saved even more money toward my Demonstrator replacement. Happy Daze.