After complaining about the backtracking and repeating levels thing on this game in ugvm, I got a reply saying that you don’t need to redo levels so much, if you explore the map and find hidden islands, and do those instead.
So, armed with that advice, I set about finding hidden islands, which did indeed provide a larger quantity of Material needed to build things (in this case, a radio tower) than the proper “story” levels. I had to redo a few of them to get enough, but they’re easier and shorter than the story levels, so that wasn’t really a problem.
Hurrah, eh? That means the game instantly stops being crap and some of my former faith in the Sonic series has been restored! Only some mind – there’s still all them horrid 3D ones…
Anyway. So with the tower built, and Johnny the Shark beaten a further three times (I have four Chaos Emeralds now), I found the next story island, the Ghost Ship one. And killed the far-too-easy boss at the end.