Both mechanics ran their own bike servicing businesses and gave plenty of confidence when dealing with them. But they were both singularly ineffective in solving the problem, neither is currently trading at their original premises however. Apart from the clutch I was very happy with the other work they did. What makes me think it is not the plates is that the problem reoccurred within days of the plates being changed (and they have been changed twice by two different mechanics!). I am tempted to get a second hand clutch on ebay https://www.ebay.co.uk/itm/Kawasaki-Z750-Z-750-Clutch-basket-plates-hub-complete-FREE-UK-POST-Z258/323424758552?hash=item4b4d9e2b18:g:9B0AAOSwbmFanXHd and replace the whole assembly. I am tempted to do it myself, partly to save money but also because I have discovered that I rather enjoy tinkering with bikes. Having read through the process in the service manual it looks straight forward.