@btlevie unfortunately the fix for me was to revert to a backup and resync square again.
@wpnomad after reverting to backup, getting everything to the same versions as previous and enabling all plugins, everything is working fine. There must be something wrong with the Square Woocomerce plugin!
I shall update when it happens again.
So as far as I can see, disabling the Theme and all plugins, as soon as I activate Square it immediately causes the error.
Deleted Square and re-added it, same effect.
I will note that everything was working fine and no updates had been made when it decided to start having this error.
