Thank you for your replies.
Your advice describes what I have done but after my restore from Time Machine the Music app cannot find the files there. Try and play a music video I get told that the file cannot be located and do I want to find it. If I do ‘get info’ the file tab eventually shows me the path to the file but still cannot play it. This made it look like the files were stored elsewhere.
I have found a work around - use Music’s import function to import the file. This then requires then deleting, with Finder, the original file and renaming the new imported one by editing out the 1 added to the file name on duplication.
One consequence of my work around is that I get an unknown error 1308 saying the Music library cannot be saved.
I have booked a call with Apple.