Thanks, bunanson and everyone else who helped set this out so clearly.

One little new thing that may help someone. I found that my usual WinSCP log-in wouldn't work, and it turned out that my tablet's IP address, which had ended with .100, had been switched (by my ISP, or by the order I started up online sessions on different computers?), to .102. All WinSCP had said was that it couldn't connect. I checked, found the IP address had changed, and just edited my login.