Talktalk email failures

I am trying to add a talktalk email address to Apple Mail on an iPad and Thunderbird on a MacBook Pro without success. I believe the problem is the same for both and may have something to do with recent issues Talktalk have had whilst updating their system.

On Thunderbird, the automatic registration of the email address fails and I have to manually configure. After correcting the settings as required by TT I get an error as shown on the attached screenshot.

These settings are absolutely correct according to TT and I have checked and re-checked the account name & password. So what's going on? Talktalk are currently referring their customers to Apple for support (as they would).

It's a similar problem on the iPad using Apple Mail, but more difficult to check. I believe there's an underlining problem but I can't nail it.

Both the iPad and the MacBook have other TT accounts working on them, with apparently the same settings!