I appear to have "circumvented" the problem. I opened network-manager-gnome as root and entered my wireless SSID to create a new wifi network, and, bingo, the wifi worked. Everything now seems to be normal - wifi automatically there at boot-up. It seems to me that the frequency of these little problems, quite independent of Debian-based distribution, has greatly increased since systemd. Is it just coincidence, or could there be a root cause there?