OK, great (if slightly embarrassing) news: I've got this working (with one issue I'll raise in the main channel). I was missing a

__init__.py

in one of the test folders (This isn't my code, I'm copying into my Pants PoC folders from our main monorepo 😬 ).. I'd previously fixed this but re-copied everything after the main repo had a release ... Anyway. All is good now, no need to modify the sources. I just needed to make sure everything had a

__init__.py