I tried it all again - paste was there with Nautilus, rebooted, same, changed to Nemo, paste was gone, rebooted, still no paste option. Here's a screenshot I took just now after copying a txt file into the clipboard.
Not to worry, I can either live with Nautilus for now, or live with no paste option on the desktop. Hopefully I won't have these issues on the xfce desktop.
This is the kind of thing that left me wondering whether some of my issues weren't underpinned by some common cause, such as ownership or permissions, or having a drive set up wrongly, which in turn led me to be more worrisome and more for people to have to put up with as I flapped around. 




