For the benefit of anyone else who could fall into the same trap, the feature update is catchily named '20H2', and may or may not be waiting for you in Windows Update (it's a phased release.)
I think the Edge browser may have failed to survive the update because I wasn't running the latest version, which you can only check if you are able to open the browser, which of course I couldn't after installing the feature update! 
You might think the update installer would check this apparently critical requirement wouldn't you? 
Can't guarantee this will avoid the problem, because having spent all day messing around with repair utilities and eventually restoring to a restore point, I'm in no hurry to take the plunge again!
I wasn't in a hurry, but Windoze was - the feature update installed itself
, and the Edge browser still works, so, if you want to be able to still use the browser after installing the feature update, make sure you check that Edge is up to date before doing so.
