So it's a bit hacky to say the least but I've found a solution - In Reality Converter I just added my diffuse map that I had exported from Blender into both the diffuse AND the emission - It looks blown out in Reality Converter / Composer & Xcode but looks right (to my eye at least) in AR Quick look and my app! If it's too bright for you, at least you now how an easy way to control the brightness (copy your diffuse map, save it as an emission map and adjust the exposure in Photoshop to taste)

It's been literally years and this still hasn't been figured out - not even a workaround. There must surely been someone in the AR team at Apple working on this. This is such an incredible collection of technologies and currently let down at the final hurdle

Also, AR Quick look's AR View tab defaults to showing my model on a ground plane (horizontal). Is there a way in Reality converter to choose to have it default to a wall (vertical) view

Scratch that. Just updated Reality Converter to latest version and working fine!