Checking the crash log I saw it had an “Assertion Failed” for the Static Mesh component. Meaning their was a dodgy mesh somewhere.
I ended up migrating my dodgy asset to another blank project, deleting different imported static meshes (All walls, all windows, all floors), and seeing where it stopped crashing, I was finally able to track it down to a single 10M long window piece.
I’ve no idea how or why it decided this was the dodgy asset. It’s been made by stretching the 5m piece out, and the even longer 15m still works as well!