Very strangely I checked App Store Updates this morning and found there was one available for iTunes 12.5.3 (see my post earlier in this thread where I mention this update had already been made automatically a few days ago) so I clicked on the update and it was duly downloaded and installed (again). The installed update was then showing as installed today on the Updates page. Tonight when I went to check the installed updates page it had disappeared.
Not at all sure what is going on. Updates are being installed twice or so it appears and the Updates part of the Mac Apps Store has disappearing installed updates. This is all getting a bit silly.