I suspected as much but the query was raised through contact with Outlook Mobile Support who are investigating why the client - in this case for Android - continually requests authentication of all the IMAP settings. It's been happening for a few months staring with requests for password and now all the IMAP settings. Originally it was occasional but now it's virtually every day, and it means BT Mail is unusable on the Outlook Mobile Client.
I'm seeing other reports of problems described elsewhere on here relating to migration from BT Yahoo to BT Mail so it may be related to that.
BT at present do not offer two factor authentication. It was asked a while back why it has not been implemented and a moderator replied that it was something that may happen in the future.
As regards your problem have you checked that you have the correct server settings.
See these links
Thanks for this.
I believe I have the correct settings but will check again.
Thanks for your suggestions.