Ok, thank you very much, Paddles!
I will totally uninstall/reinstall both sceneries, along with all the others I am not using. I fear there may well be an interference with another scenery perhaps. I will go through the Scenery Library and totally uninstall rather than just uncheck, all the unused scenery folders.
Thanks very much for the advice. I don't know why I was just unchecking, when in FS9, which I have used since '04, I always uninstall from the Library entirely, then remove the folders to a back-up folder.
I'm off to clear my Library! Thanks again. I'll get this FSX-SE down eventually. I picked it up in January, so it's still a bit new to me, and some of the details of How-to and What-to, etc, are different than FS9, so...I learn as I go! That's why I tend to ask a lot of questions. It's the best way I know of to learn more rapidly

Pat☺