I did have an ANT+ dongle plugged in.
Pairing screen with it plugged in - stuttering, almost crashing.
Unplugged the ANT+ and it immediately started responding again. Plugged it back in - no problem.
Closed the game down, ANT+ still plugged in, and relaunched the game.
Issue has gone…
Zwift config or ANT+driver must have had something corrupted which was resolved by removing and re-inserting.
Thanks for the suggestion Arend.
Hope this assists others should they ever run into the same problem.