it suddenly stopped working.

Please explain in more detail what that means.

I stopped reciving emails in Thunderbird on 01/04/16 even though I can see them elsewhere. Tried changing settings on my account from SSL/TLS to none as advised by Talktalk but made no difference. Deleted my account an T'bird and tried re-adding but it fails as it seems it doesn't recognise dsl.pipex.com as a valid ISP address. (Pipex was absorbed by TalkTalk years ago). Never had this problem until 01/04/16.

But that's not the address of a server. It needs to be pop.dsl.pipex.com (incoming), or smtp.dslpipex.com (outgoing).

Pipex used to a reputable ISP with high standards. I'm amazed that they don't offer SSL or TLS, and that they use "pop.dsl.pipex.com" for both POP and IMAP.

Tried changing settings on my account from SSL/TLS to none as advised by Talktalk but made no difference.

If they advised you not to use encryption find a new email provider. Simple as that. Any email provider not offering SSL/TLS these days should be out of business.

Yes I agree. I have had SSL for years. The point is it doesn't work either way. When trying to set up my account afresh in thunderbird I enter my entire email address which ends with @dsl.pipex.com and this is the bit thunderbird appears not to recognise as it searches for ISP's.

If Thunderbird fails to automatically detect the correct settings try 'Manual Configuration'. https://support.mozilla.org/en-US/kb/manual-account-configuration

You'll need to have the correct server settings available as per your email provider's instructions.

Tried that, still no good. I strongly suspect TalkTalk have changed something which means I can no longer receive my emails in Thunderbird, though I can get them in Outlook. TalkTalk website was closed for maintenance on 01/01/16 and although it was available on 02/01/16 it isn't now.

01/04/16 I mean not 01/01/16

Did you get an update to Thunderbird around that time? If so, it may well be your anti virus that is blocking access to the server.