Majestic scene. I think though that your clouds may have some more softness or grander scales, and speaking of clouds; something you see here - it happens to my renders too - the clouds themselves are grainfree, but the shadows have a lot of grain (front area). I wonder why that is so. I also often notice it in reflections of clouds on water, e.g.
QuoteWonder if there is a better way than I took to localise the openings in the sky
You could make a black square with a soft white dot, covering almost all the square (or use this one), and project that (image map and camera) towards the sun. Place the camera where you want no cloud shadow. Take values form sun angle and height to feed camera, and set angle. You also have to invert the mask before connecting it to the final density input (or mask the cloud fractal by it). Takes a bit of experimenting...