are they actually in your talk talk inbox, or in a spam folder somewhere?

Thanks! It had not occurred to me that messages I had sent to my own computer would be treated as junk, particularly as some got through, but that is where I found them, all present and correct.

With the benefit of hindsight I assume that Thunderbird's spam recognition software was triggered by receiving so many emails from the same source on the same day. Some got through before the trigger was activated.

you are in all probability dealing with two spam filters. Talktalk will probably have one as well. It may well act as you suggest. The Thunderbird one to the best of my knowledge takes no notice of the from address. it is usually spoofed by spammers so is somewhat irrelevant.

Thunderbird places a flame icon beside what it thinks is junk/spam talk talk will not, even if they both place the spam in the same folder you may be able to determine which is acting using that icon.

More information on the Thunderbird junk filter here. https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages and here http://kb.mozillazine.org/Junk_Mail_Controls