Thanks
I'll get in touch with BT and see if they can help. I can't see Google being of much use!
BT wont help you with gmail.
I've sorted it!
I deleted my Gmail account and then re-added it. It was very straightforward, once I'd put in my BT e-mail and password (and server name?) all the other boxes in incoming and outgoing settings populated themselves automatically. And, whilst I did lose everything in Sent, Bin etc my Inbox came back intact.
So far so good, but then, when I tried to send an e-mail, nothing happened. So I took the advice on the internet and cleared search history, cache etc from my Chrome and Samsung Internet browsers. This did the trick.
Thanks for your help gg30340.