A strange thing happened when I went offline on my PC whilst Twine was running.
Twine suddenly went from the latest version down to Twine 2.2.1 and to SC 2.21
I tried to manually move it to a more modern version but the best it will do is to go up to 2.28 via the Twine interface.
SOLVED! It turns out I had a version of Twine I had to uninstall.