That wasn’t quite sufficient. Last week, I had removed all versions of Python on my system, then reinstalled Python 2.7 directly under C:. I noticed this morning that the reinstallation of PlatformIO was also installing version 3.7 under the .platformio subdirectory.
So I uninstalled platformio again. Uninstalled all versions of python as I only need it for platformio. Reinstalled platformio. Updated my system level path to point to python.exe under the .platform directory.
Now, all seems to be working again.