As a follow up to this, I have now, eventually, created a new database and the "Vertical" keyword is now showing 3438 images :-)
However, although I made sure only the ADD for my new database was selected, as it was building, the message on the metadata update in the tasks window said I was updating 2 of 2 databases and the catalog.pmdb file for both databases was being written to.
While building the new database (27,000 images) it froze during building previews (actually only during the final 3000 files). I did a search here and found a suggestion to run a full sync to force the preview build to restart, but PM6 was locked solid, and I needed to force quit.
After restarting, the preview build automatically restarted, and after 3-4 crashes and restarts, the previews were completed :-)
it seems to OK now, and I will delete the original database.
As an aside as my rebuilds and restarts seemed to affect both databases i.e. the message that metadata was being updated in two of two databases, I did check my original database, to see if there were any changes, but this is still only seeing 27 images with the "Vertical" keyword.
Cheers,
Graham