Thanks again, TT2. I feel really stupid, but I must be doing something [simple] wrongly. Please bear with me...
I'm closing iTunes, then opening them again as suggested whilst holding the shift key down. I select "choose iTunes library" and get a dialog box headed "Open iTunes Library" with 6 folders and a file at the bottom called "iTunes Library." Sure enough, when I hover over this file, it says "Type: iTunes Database File," "Size: 3.41 MB," and "Date Modified: 15/02/2024" [today's date]. When I double click [?] on this file, iTunes re-opens but it is still showing the previous version of the library. Any ideas what I'm doing wrong?