Mate, you’re onto something proper rare. The color Steamboat Willie is top on my find list. Been scouring old BBC tapes for years with no luck. Don't suppose you've got any screen caps or anything? Even a snippet could help verify its existence for the others doubting in the comments.

Collector here. 'Captain Starshine' has been on my white whale list forever. My best find was a mention in a TV guide from '89. If you get a lead on a physical copy, you'd be a legend. Keep us posted!