BT-Yahoo proved to be impossible to contact. The BT part offers no telephone numbers to call (except to sell you things and they are closed for the weekend) while Yahoo's log-in system has my Flickr password as the default which is not what I gave BT three years ago.
So I researched the internet and found smtp.com. Not as cheap as BT-Yahoo, perhaps, but actually much more reliable, it would appear, and the help line was open over the Easter weekend. The good news it works, which is all I ask of it.
My next problem will be to cancel the BT-Yahoo service. That could prove to be a lot less easy. Can anyone provide me with their phone number?