EPILOGUE

apt -f install and apt autoremove both reported that everything was fine and that I was up-to-date (except for more than 20 packages "held back").

Sometimes I find Linux troubleshooting a useful mental exercise. Not this week.

Downloaded the latest KDE Neon onto a USB stick.

Backed up and re-installed without reformatting.

All seems well though I need to bring back the apps I had.

Thanks to all who helped.