I followed your advice. Removing the path to Python 2 made the URL checker work. Removing Python 3 and keeping Python 2 didn't.
Then I deleted the bs4 folder and used pip/pip3 to install Beautiful Soup again. Once I'd done that I was able to run the plugin with either version of Python. That's great because I have several plugins now and some need Python 2, some need 3 so I have to have both.
The plugin does make life a lot easier. Thank you for writing it. I really like how the errors appear in the validation window at the bottom, like they do when running FlightCrew.
What I would find helpful though is to either: keep the window open with all the links scanned so it can be cut & pasted into a document OR offer the option to save to a text file too. Being able to navigate directly to the broken links in Sigil is marvellous but it would be great to have a definitive list of all links. I format books for a small publisher and I could send a file of broken links that URL Checker found.
I hope that makes sense.
I really appreciate your help.
Mr B