I work for Sony Music in the UK and I'm trying to licence the theme tune from Happy Days to a new multi-artist compilation. I have the Pratt & McClain version but I don't think this is the actual TV version used in the sitcom. I'm sure one of you would be able to tell me who performed the TV show version and hopefully which record company owns the rights?

I do know that the (second) theme was composed specifically for the show, which was produced by Paramount. Not sure if Paramount still has a separate music division, but if not, the rights are probably held by Viacom (Paramount's parent company) or one of their music subsidiaries.

The mystery singer on lead vocals for the "Happy Days" theme is Hollywood session singer Ron Hicklin. Hicklin is well known to fans of "The Partridge Family" as one of the professional studio voices who sang background vocals on their records.
Rights to the song would certainly be traced back to Paramount Television. Check out the TV Toons CD "Television's Greatest Hits 70's and 80's" compilation which features a fairly good stereo master of the 1980 version of the song. According to the liner notes, the song with music by Charles Fox and lyrics by Norman Gimbel, is owned and published by Bruin Music Company (BMI).

And it turns out that Bruin Music Company is the current name of Paramount's old Famous Music label.

IsRon Hicklin the one that sings "Goodbye grey sky, hello blue....", or the part where they sing "happy and free" and "share them with me", because i think the "Goodbye grey sky..." singer is Potsie ( Anson Williams0 , or it might be Ron Hicklin

Anson Williams never sang the song, he only Mimed it. The lead vocal is by Ron Hicklin.