Solved.
I bought the GOG version of Skyrim SE and I'm glad I did. I have had countless problems with Steam, every time they "upgrade" Steam there is a chance that they will bork my installation. Which they have successfully done for Skyrim LE and now SE. Great Job!

I run Linux and have got used to doing everything manually and what I need is the location of the Data folder so that I can try and get my favourite mods installed. Reading through the posts here they may not work but playing without "Skyrim Community College" does not bear thinking about.

Any advice will be gratefully received.