Yes, I will try some of the suggestions and have contacted BT to make them aware of the problems and the fact that they are being discussed on this and other forums.
However, I am not going to try the more technical workarounds because I would not know what I was doing or be able to keep track of any unintended consequences !
I tried resetting the modem, on the advice of BT, which resulted in one of my machines updating but not the others.
For all I know, that might have been a coincidence, I have no idea.
It seems that MS have admitted that there is a problem at their end anyway, which is the main thing to know, from my point of view.
Glad that I am not a large business, running scores of machines !