I will bear this in mind in future as I am sure my future son-in-law my go-to Apple expert will be able to figure it out. However, as you predicted, I am again able to send send through BT Mail although I have no idea why I couldn't send mail or why it has started again .
I have had similar isolated incidents of this nature in the past and being retired now life is too short to spend valuable time trying to sort issues like this out so I will still review whether to remain with BT when I get back home.
Thanks to all for the advice. I hope I will not have to visit here in future.