OK - solved. This update is part of the process of preparation for offering the Windows 10 upgrade. I had previously hidden KB3035583 which is the one that puts the Win 10 Upgrade icon in the Notification area....WU should have known that and not kept on offering the above. Despite saying it was successful 6 times and showing in update history, in Installed Updates, it isn't there so was not actually installed at all. I've now hidden the offering along with that other update, until such time as I want to accept MS's upgrade offer. WU is no longer saying I have an update waiting. WU is weird at times and very frustrating.