/Library/Frameworks/Python.framework/Versions/Current/lib/libpython3.9.dylib
Changed "pylib_mac" in user.json to point at this file, and it works fine. The dylib file above is symlinked (like the Universal installation) back to 'Python' in directory /Library/Frameworks/Python.framework/Versions/Current - still not sure why, or why it causes the problem I have had with finding the Python library.
The Options Editor in Cuda was insisting that the path to search was for Python 3.6 and I could not change it in there, but entering the above path for the dylib file by hand in user.json has fixed it I think (the Plugins menu is now populated and Cuda starts without any issues).
Now I can get on learning how to use Cuda, and maybe be productive!

Thanks for all the help
Tony