Once again, thank you all for your support.
I've backed up my profile Milo and tomorrow morning, feeling fresher, I'll follow the Standard Diagnostic procedure from Mozilla and will see what I achieve and then take things from there. Never having had this sort of problem with Thunderbird or Firefox, I didn't know that this procedure was available and had been published.
I'll certainly take your advice on board mixture. I see the benefit of the junk handling (even when it works as it says on the tin) in Thunderbird as being pretty marginal so I'll probably ditch it for the future. I hope that my lost e-mails (having spotted the senders on a few more this evening, just as they disappeared into the pit, I saw that they absolutely were not junk) will be recoverable from some dark corner of my hard drive. My choice has always been that my e-mails are scrubbed from the server as I download them but I may review that now.
I must say, vulcanised, that I can see some merit in your approach as "better the devil you know...." but I have always gone the other way and installed every update as soon as reasonably possible because I feel that better security generally comes from that approach.
I'll post a report upon my success or otherwise soon.
Thank you again.