Also, forgive me if I'm wrong but when we talk about no true overcast, this to me looks like it is; I realise its 'fog / haze' and not true 3D volumetric clouds, but it'll at least give the same feel descending into think fog onto the runway right albeit at a very low altitude?

I would say, most definately a mountain shaddow. The sky texture in the background suggests the picture was taken with the sun very close to the horizon. If it was a cloud shaddow, it should extend much more to the left and not stop right under the cloud.Edit: with the number of clouds you can see, there would also be more shaddows and not just this one.

To the true overcast post: The only difference is that you don't "break out" in fog. The visibility stays low. With an "overcast" layer you wouldn't see anything below until you broke through the cloud layer.

looks like a crayon drawing!