Thank you Cliff,
The issue seems to have resolved itself. Yesterday I created a test document on my MacBook Pro, edited it on my iPad and then on my iMac. No problems.
Then I opened older documents, that before had the 'read only' message on my iMac and wouldn't autosave. Now they are working just fine.
I suspect something happened at server level, as the OS, and the versions of OneDrive and Word that were previously affected are now okay. I hope everybody else finds their documents working too.
I'm glad this issue didn't decent to the old days of a blame game between Microsoft and Apple - let's not go there, please.
Best wishes