At this stage I started looking through all of the NS&I help files, and I eventually found
Your phone network could be automatically blocking the call or sending it to a junk voicemail folder. To unblock the call, you may need to add the following numbers to your networks VIP list:
08085 007 007
But what is a VIP List, and how do I enter data into it? So more searching on the BT site - and I found I could carry this out by dialling 1572, and selecting the appropriate menu item - which I did.
Now back to NS&I, and this time I received a phone call with a one time pass code, and on entering it, I moved one step further. I was now required to create and enter a (new) 6-digit PIN, which now joins an ID number, a password, and a list of 6 responses to questions that they might ask. And I assume that they will now have logged this computer as part of the authentication. And presumably I will need to go through all of this if I want to use another device (PC, Mobile Phone or Tablet)?
And a final complaint, why is NS&I using their phone number 08085 007 007,when they appear to know that it is likely to be blocked by the phone network? I have received automated calls with passcodes from several other sources - and none have been blocked.