Quick update on this - should anyone have the same problem again.
There’s definitely a bug somewhere… I cleaned out the Intermediate and Saved folders rebuild lighting and it works fine in PIE on my laptop. If I package it, lighting breaks as soon as I run the .exe!
I copied the project (with lighting already built) to another computer with UE4. Packaged the content on there, lighting is fine when running the .exe!
It would be great to get to the bottom of why the packaging is breaking on my laptop…?