Just as an update to the above, I followed some advice I read elsewhere that following a browser update, it sometimes wasn't enough to simply log out and log back in again to chrome. The advice was to go into the Chrome Settings menu and select the Reset Settings option, which would return settings to their default values.
I've tried this, as I had nothing else to lose and, much to my relief, everything is now working perfectly again. No more html showing when editing text content.
I suggest clearing the local cache for the browser.
Chrome is dreadful at holding on to out of date stuff.
Ctrl+F5 will load a fresh page.
Ctrl + Shift + Delete to call the Clear browsing data interface.
Select the cache files you want to delete.
Many thanks.
A simple refresh did the trick.
I was using Edge. The strange thing was that the test system looked fine.
Must be something to do with a Windows upgrade - all looked as usual on another PC but the upgrade hadn't been done.
Nope, it's browsers and the internet.
There is a thing called Edge Caching and combined with the browser cache, work well for many sites BUT low volume sites (number of visitors) suffer as the interaction between the two can be a bit "slow" and pages can get "out of date".
Forcing an cache update fixes this, till it happens again....