Used the number provided by imjolly (thankyou) and got the very friendly and efficient Pete, who had access to our old account. We're now back on BT - or will be on August 28.
Many thanks to all you good folk 😊
Having got back on to the BT forum (thanks again!), I'm seeking some help about emails. Only one of my email addresses has reconnected - the one BT gave me originally. I had created a separate one to protect the "base" one but the one I used most "can't connect to the account". It's so long since I had to reconnect I am - to use an old word - flummoxed and going round in circles.
I would be very grateful if someone can refresh the way to rescue many years of messages.
Working with MacMini, MacMail, OS Mojave v10.14.6.
And yes I do feel a bit stupid 🤔