Is talk Talk using Microsoft's hosting for their EMail by any chance? That sounds a whole lot like an issue with Hotmail/Outlook using POP mail accounts. Google appears to be telling me it might be AOL.

Using the Hotmail issue as a basis for guessing. Check the Talk talk web mail for a setting that moves mail deleted by mail program to a POP mail folder under the deleted folder. Change that to allow email programs to actually delete mail on download.

Thanks for the feedback, Matt, but I have been on the Talktalk community pages this morning and found that this is a common problem that began on 24th February for all contributors (and me). The community is scathing about Talktalk claiming that PoP 3 is the problem, when IMAP has been affected as well. Talktalk claims that it has not introduced a fault but has not explained why different E mail clients are affected or explained why it thinks loss of E mail histories is acceptable.

I am unable, maybe naively, to access "change settings", as explained by the Talktalk "help" page because I can't access it where the help page describes it to be.

I think I'll set up a new (non Talktalk) E mail address.

The fact that it affects your iPad does rather indicate this is a server-side issue and not specific to Thunderbird.

That "settings" page you are looking for; is that on the talktalk website, or supposedly in your email client? If the latter, maybe we can help you find it, if you tell us what setting you're looking for.

Yes, I have accepted that it is a Talktalk problem. The settings issue is also Talktalk.

Thank you anyway.