Thanks, Jo.
I had a bit of a scare earlier this week. The working file stopped . . . working. My worst nightmare. I spent a day tearing the scene down, trying to find the culprit, but it didn't seem to be anything in particular. Once it reached a certain level of development, it would crash TG on open. No delay, no warning.
Ended the day feeling very disheartened. (There goes a year's work. Oh, well.
) But the next morning something told me to check out the painted shaders. There were several in the scene. A forum search turned up several references to painted shaders causing problems similar to mine. Turns out they are best used in moderation. So I spent a couple of hours converting them to images, and the scene is now up and running again.
I keep daily backups, so in the end just lost a couple of hours' work.
New rendering of reconstructed scene with a few more details and an early cloud test. Altocumulus Castellanus, out of the box. They have a nice effect on the foreground-background contrast of the lighting, which is now very close to where I want it.