Thanks for the ideas.
@Njeneb. I didn't realise I had forgotten to check receive shadows for the clouds in front of the hills - certainly makes them more natural looking with that on, but now I have to adjust the cloud density a bit again.
The quality of the clouds actually looks rather good in the full render - I always set them to quality 1 which is what I keep seeing recommended, and I did a test of a small corner through the night that looks fine in full size, so I'm not convinced I need to up the samples there. I'll do it and make another test to compare, though.
The image here is just the quick render that took 30 minutes, so it is not fully representative of the final image's quality.
Making the lake water smoother would reduce the render time quite a bit. I'll see about how it looks with a smoother water shader for the low terrain too.
@Dune Yes I know I should start with the basics and work on terrain to get a proper feel for that before working more on water and clouds, but it's just too much fun making a finished picture that once the terrain looks okay I just have to add some water and clouds, and so I only learn a little each time, I guess, but I enjoy it more that way.
I certainly will read through earlier posts and search for different subjects, as I already have started doing every time I wonder about something, and I pick up bits and pieces here and there.