The diversion to intricacies of DNS configuration didn't get my any closer, nor did reboots and lots of Googling (lots of people get these errors, but they go away once they fix DNS or Internet connection issues, neither of which I have).
But I have found a clue that I don't know what to do with:
If I stop the ntp service and run ntpd manually, it resolves the iP addresses correctly: sudo systemctl stop ntp sudo ntpd -g -u 109:113
If I kill ntpd and restart it via systemctl it fails again. So it is something to do with how it is being run as a service.
I'm still stuck but in a different place now, any suggestions?
Mark
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