Yeah, that's a fifty piece ensemble! I checked out a live concert on YT of the 21st Century Schizoid Band in Japan. and that seems to be the only place where he wrote arrangements where he and Mel Collins could play jazz sax solos on a few of their songs that lent themselves to jazz improvisation. His playing is fine, but it lacks the spontaneous feeling of the 1969 concerts. Mel really impressed me with his playing on a couple of his features. Michael Giles was the real instrumental star of the show with his constant fills. They couldn't capture the crisp sound of his snare drum that he had on the first KC album, which was a shame, but I don't know whose fault that was.
Although the 21st CSB can degenerate into nostalgia, it's probably the only place you can hear Ian on alto playing jazz. The only other choice is to go searching through the 26 CD(!) Box set of KC, when Ian still saw himself as primarily a jazz sax and flute player, influenced by John Handy and Eric Dolphy, as he said in his press kit. They feature interviews with each member of the band on the YT 21stCSB Japan concert,, and Ian says that he sees himself more as a record producer than anything else, and I guess that sums up why he never pursued any semblance of a jazz career.
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