Further to the above, I have found two other posts describing something similar to my situation:
https://bugs.launchpad.net/calibre/+bug/1648589
https://www.mobileread.com/forums/sh...d.php?t=350064
I'm fairly sure, now, that the issue is nothing to do with Calibre; the files do transfer to the Kobo but, for some reason, something goes wrong when the Kobo tries to process and add the new books to its database.
I have tried installing NickelMenu, as recommended in the second link above, but it didn't work either.