Here is the solution, discovered by trial and error.
In the file called ‘gemfile’ in the jekyll directory, there is the following line:
gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]
Removing this line removes the problem
So it’s to do with the WDM ‘watch’ service. But this is inessential, so it functions without it.
The docs suggest substituting a slightly different line which I think is intended to do the same thing. It doesn’t work either.
Should I perhaps raise a query about the fact that WDM doesn’t function?