I missed the "it's never had a nice gearbox" since picking it up... my fault. change dealers, don't settle for what one dealer tells you. call Kawasaki parent company not kaw uk.
I do stand by my statement, we have to be responsible for our use. people bang gears, over run the rev limit, up shifting. over downshift (no protection from rev limiter on downshifting too far). shift forks, and arms get bent quite easily (especially with quickshifers), and once it starts, it will not fix itself and only get worse.
and as I stated, I wish him luck getting this sorted. I don't want to see him not get it fixed through kaw, I truly hope something can be done.
maybe work a deal with Kawasaki that you will pay to authorize teardown (at a dealer of your choice), to inspect and verify what is wrong with it, and if it is abuse, wear related you will pay for the diagnosis and repairs... BUT, if it is found to be a mfg issue, kaw pays for all of it. then find one of the dealers you spoke with that says there is a problem, and have them help you with the process, and kaw.
good luck! keep us posted on what you, they find.
May we all, get to have a chance to ride the fast one, walk away wiser when we crash one, keep hopin that the best one is the last one.