Thanks for your reply.
I wish I had more useful information on this, but...
Since the last thing I had done (out of idle curiosity, too) was to add some new boards via the Board manager, I went back and updated the definitions for my board.
After that, compilation went as before and I could program the board.
Only remaining problem is (off-topic alert) that the 30 or so ATMEGA328's that I have ordered from Digikey over the last year (using the same order-code each time) have chip-ID's ending in both 0x14 and 0x09
Thanks again