Finally, I can see my new card in the Barclays app. It's only taken three weeks (or so) and a number of calls and a branch visit. The missing link, it would appear, is that the card had to be 'registered' - and only then did it appear in the app. This meant I had to set up a new online BCard account with a unique username that had to have one digit; a unique six digit code that couldn't have three digits the same, a memorable word (from which you extract random letters on the basis of position) and a phrase to remember the memorable word. For crying out loud, no wonder it is easier to use an app with facial (or fingerprint) ID.
Anyway, I can now see that I have hit the £3k spend threshold and that the 25k avios will be transferred "on the second working day of the next month" ... hopefully this will be 30k thanks to HfP