Testing again I did in fact get this working! It works when you export using openGL 3.1 !
I also tried Vulkan but it was a mess - the meshes behind were always popping in and out 
I also tried GearVR and it worked there as well. However very occasionally I did also notice a small pop in and out from the meshes behind the mesh that was in front of it so it still seems just a little buggy?
Once this is reliable this will be HUGE for mobile VR! Out of interest does anyone know if Unity already supports this?
PS: A nice little side note - android build times seem much faster in 4.20!