The box has been set to factory defaults, but no change.
There seems to be no effect on using the broadband connection, browsing and email fine, and speed-tests running at usual level - 68-ish Mb
It is a new router bought this week, and has shown the symptoms since installation. Replacing the Fritz!Box with my previous router saw everything back to normal.
BT (poor) -> Zen (excellent) -> O2 (started well, went downhill -> IDNet (No complaints - but 100GB cap) -> Zen (unlimited-and now ipv6!l)