Thanks Hugo,
I think I've been an idiot!
I was half way through the process of making the suggested search. I then realised that I had failed to take account of the fact that while my administrator id has an individual subscription to the sub-forum in question, I also have a "test" user and that is the one with the faulty digest subscription. At the same moment I realised that a few days ago that I removed the "Member" group from that user as part of a test I was doing and, of course, you need to be in the "Member" group to see posts anywhere on the forum.
So I'm 99.9% sure that things are working as they should - with just one of those little untidy idiosyncrasies that because of the retained digest subscription, my test user gets to know that posts have been made but not to see them. Obviously, I should have unsubscribed my test user from the digest at the time I removed it from the "Member" group to keep things clean.
I have reinstated my test user to the "Member" group. If this doesn't return things to normal, I'll report tomorrow. If you hear no more consider the issue resolved.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.