Thanks for your reply, Tony.
My son Martin has a Windows laptop and he found that he could see all of the photos on his machine when he loaded up the flashcard in Windows Explorer. Many of them were in a folder with a title ".trashes". (The first digit in the title was indeed a dot!).There was another folder structure inside the "dot" folder. The subfolders within had titles of the form attributed by the software of the old canon camera I'd used back in 2002 and later. The filenames were of the form: "IMG_1462.JPG".
My son copy/pasted some of those photos from the .trashes folder into a new folder on the flash drive titled "Test1".
He gave me back the flash drive and I then found I could see the files on my MacBook pro in "preview" but still not in "Photos".
I then double clicked one of the "invisible" image files and it opened in preview. I clicked on "share" and I was able to transfer it into "Photos".
The basic picture information had carried over into Photos including the date and time.
Success at last! I shall now have to recover them with my son's help and shift them in the same way.
Tony, I assume you are right. The pics are probably in an old version of JPG which my Apple MBP can't read. It worries me that this king of thing might well render files unreadable in the future. Perhaps I'd better start printing everything!
My thanks and best regards, Patrick