I’m making progress… with some help from the forum 
I forced the section of the upgrade relating to Debian Bookworm using
yunohost tools migrations run 0027_migrate_to_bookworm --force-rerun --accept-disclaimer
This was based on @tinder’s post here (Migration from 11th to 12th fails) and @alek’s replies.
This seems to have fixed my issue. I’ve created a pull request on the docs containing some of the extra information that helped me and others during install. It’s here: Update index.mdx by kathep · Pull Request #2618 · YunoHost/doc · GitHub
I’ll leave this thread open until I’m sure I don’t have other heinous issues as I go through the next stages of upgrading (re-adding metronome etc)