And Rubberduck saves the day.
This morning Word has been very truculent, refusing to play ball when adding or changing references in the VBIde. I had to rebuild some .dotm template files from older backups (only a few days ago) When asked, Rubberduck pointed to a function in the Layout Template for which 'NO VALUE WAS RETURNED". The missing return was non obvious and as it was buried in a library I would have taken some time to find the issue manually.