Thanks for the offer but it’s too late, David, and I suspect not a GP/GP+/GB problem anyway. I suspect Gutenberg plugin, which I downloaded many months ago on advice from somewhere, without any obvious warnings that it is a steaming pile of experimental excrement.
Now I’ve been royally rogered by this thing. I compared an old copy of the WP database with the current one, and it’s full of changes and deletions, even though I have made no edits to the site in months.
So the WP site is deleted, goodbye WordPress, and I’ve moved my phpbb forum into the root. I’ll republish all the WP pages as threads in the forum. In a way it’s actually neater and easier.
Lesson learned.
I do miss the old days when one could hand-code html files, and everything was fast, maintainable and simple.