Sorry, that was the desktop version number. I was seeing the same issue in all platforms, web included, and was getting desperate.
In the end, I managed to resolve this through weird combination.
I duplicated my main todo note, but the new one had the same issue - no tasks. So, I trashed the duplicate.
Evernote support suggested searching for the Tasks in other notes and I found that in the Trashed note all of the tasks were visible!
When I restored the note, they were still there.
Trashing and restoring other notes that had the problem resolved it for them, too.
Thanks for your suggestions.
Ian