So you don't normally allow Thunderbird to store passwords?

No, we both have been drilled to NOT leave passwords anywhere. Thunderbird has worked for us in this format for many years then, all of a sudden it doesn't seem to. If we had had an update, or even an update for Microsoft 10 we would blame that but, to our knowledge, no updates have been made. We did receive an Email from Talktalk to ensure we had SSL enabled, I tried the method TalkTalk gave me (my Wife didn't) but then I couldn't receive Emails (my Wife sent me a test Email) so I reverted to the original settings, BUT under both settings I still got the password request when I opened Thunderbird Whoops!

I have lots of passwords. Too many to memorize. And writing them down isn't safe, so I store them in encrypted stores. Where do you keep yours?

In Thunderbird, go to Tools|Options|Security|Passwords→Saved Passwords and make sure it hasn't saved any passwords. If it has, perhaps you'd like to remove them.