I sorted this out; I had 2 problems.
Eventually after a day or more the action timed out with an error which pointed me to my first problem.
- An old version of TryGhost/action-deploy-theme
- When this didn’t work I realised there must be something wrong with my secrets. It was then I realised I was storing secrets in the wrong place; in “Codespaces - Repository Secrets” instead of “Actions - Repository secrets”
Now works as advertised :-)