Thanks for those useful instructions coenoby (l've saved them 👍).
I'm holding off changing anything just yet as I have two (now hardly used) ntlworld email accounts. One appears to have corrected itself (can now send / receive), but the other has the message "Incoming server login error: NO Authentication failed" when using my AquaMail app. I'm able to send email from the second faulty account, but nothing receives.
Hopefully, as one account "currently" works (no passwords were changed), I'm hoping the eventual fix of the system will bring back the second account. If not, I'll try the app specific password as you've kindly detailed.
Cheers