I'd suggest you look on Belle's Domain. It has more information on Starlight Express down the years than anywhere else I know.http://www.bellesdomain.co.uk
I tried to get the score for Belle's Song some years ago for an audition, ended up writing it out and scoring it for piano myself. PM me if you'd like scans. The official score simply doesn't exist as far as anyone knows - my attempt is strictly amateur and strictly NOT for any kind of commercial use, of course.