Well, it looks as if I've found all of the e-mails junked in error over the past few months. On the basis that they appeared on my computer, albeit on very briefly, I thought that they must be somewhere in my Thunderbird profile so I started looking. Under Mail there is a Local Folder file and a subfolder with my name on it. Between these two I can find all of the mails which remain extant upon my Thunderbird opening page. Then there are six more subfolders "pop3.terra-1.es" up to "pop3.terra-6.es" and then simply "pop3.terra.es" (terra being my internet server and .es referring to Spain). All of these are essentially empty except for number 4 whose "Junk" subfolder contains all of the mails which disappeared - but in a very untidy (for me, anyway!) form because there's a pile of extra content with each which must be routing information or something similar.
Recovering the text of these to my normal Thunderbird front page will be very useful but I want to stop it happening in the future.... Perhaps someone may be able to advise on a simple way to do this.
Each of these folders also contains a DAT File called msgFilterRules.dat which I can open with Firefox. I mention this because perhaps it might be appropriate to tinker with the contacts - but for this I would need expert advice. Conceivably this might allow effective bypassing of the Options and Account Settings (in the Tools menu) which ignore any attempt to change the Junk Mail filtering rules.
What do you think?