Thanks for the pointer - I missed that, and my searches failed to reveal - maybe because, at the outset, I hadn't appreciated that systemd-resolved was involved.
Now, why were my older installs not affected by the problem? Perhaps some change over the course of time? Why, when it's clear that it's known to cause problems?
Ah well, my problem is resolved now and I can move on with my Python/Kivy development - all credit and thanks to you. Perhaps this thread will help others in the future.