To anyone else reading the above question,
The 'answer' to my 'problem' was solved by deleting my 'btinternet.com' e-mail account from my iPad.
You can 're-install' it (as I did) using the 'Yahoo' option, instead of the 'Other' option,
Go to: Settings;
Go to Mail, Contacts and Calenders;
Select ADD another account.
Enter your previous Name, E-Mail Address, and Password, and Description (you can call it what you like), in the box provided.
You SHOULD then see the word 'Verifying......' after you have selected 'next', and then after a few seconds you shoud see four blue tick marks confirming the 'verification' of your details.
This is what happened the first at least eight times I re-installed my account in 'Other' after it had stopped working, and whenever I tried to SEND mail from ***********@btinternet.com, or REPLY to mail sent to me at the same address, I got the the error message:
'Cannot Send Mail',
'The username or password for SMTP: ***********@mail.btinternet.com' is incorrect'
when the username or password were NOT incorrect.
The next four or five times I tried to re-install the account I did not see the word 'verifying....' OR any of the blue tick marks, but I still received the e-mails again that were deleted when the account was deleted - though I still could not SEND or REPLY.
Hope this helps
Bob