I have sorted it at last. I took my photos library from a time machine back up before Catalina. I copied it to an old iMac (lucky to have one) running El Capitan and made it the new library for the empty photos app. Then I imported all the photos I had taken since last October. I have now recovered all the photos that the Catalina new Photos app migration lost.
It was a lengthy process as I have over 8000 photos and had to go through month by month back to 2002. I found 200+ really important and sentimental photos that Catalina had just lost.
I will now back up this new composite library and sync to my phone and iPad using iTunes in El Capitan. Then I am going to let Catalina convert this library again and then I will know just how many photos it fails on.
I suppose I am lucky that I have this spare machine to do this on, but it has still been a day's work to work around something that in their own words "it just works". Well heh! surprise! "it doesn't. Probably don't care as already had my money for several computers , phones, pads, watches.