Hey @James_Robertson,
I was just having a similar problem a week ago, turns out the bench gives this error for all DB related errors.
So, My suggestion would be to check the logs of mariadb for any other errors.
I have also Documented my experience on a similar thread.
I would like to request any bench devs reading this to make the DB error reporting (in 'bench new-site' command) more transparent and let the user know what errors are happening under the hood.