I certainly wasn’t going to leave the game “done” at that, was I? I mean, 50.4% complete, even though I’ve seen the credits?
So I set about getting all of the remaining green crystals, then completed the rest of the unlocked secret levels, before racking up 10 gold cups for some of the mazes (the first 5 in normal mode, and the first 5 in mirror mode). I also unlocked Space A and Space B, and so completed them.
I don’t know how to unlock Secret 15. For each of the other secret levels, you have to collect three green crystals. But I have all 45 now (Space/Mirror/Secret don’t have any), and still no Secret 15.