OMG it's Dom! Where have you been? :x

@ Dave

I'd say the parti_volume looks much better and is more accurate than the volume primitive. The parti_volume is however, much more expensive to render but at least the objects cast raytraced shadows into the volume and it respects the light decay. In this case I used a refractive mia_mat shader on the cube incasing the parti_volume(because I felt like it ). The light is a really thin area light that's using the physical light shader in its "custom shaders" slots, I could increase the area light samples for a smoother volume but you get the point.
And no, you don't need to use the physical sky in order to use the mia_material.