After I had problems with MLAT the last few weeks after my move (it only ran for the first two or three minutes), I took a moment to think about what I had changed in my configuration. And that was the change from an SD card to a USB stick in my RPI 3. And it’s one that’s pretty lame.
So I rummaged around in my desk, found a reasonably recent SD card and used the excellent rpi-clone and the command
sudo rpi-clone -f sda
to transfer the data from the stick to the SD card. Removed the USB stick and rebooted from the SD card: Voilà, MLAT is green. No further config needed. Presumably the lame stick has messed up the clock.