Thanks for the comments everyone; glad you're all enjoying it.
Greyface, when you complete a level in a computer game, you can't usually backtrack to explore previous levels. That's what I was going for.
Establish careful rules for the player that they come to understand and expect. Then see 'em squirm when something goes wrong.
Glad you're liking the difficulty, Dave. That's always hard to get right. I kind of think there are actually two versions of 'hard'; there's hard for the people who aren't great at platformers, and there's hard for the hardcore platformers, like myself. I think there's about an equal balance of the two on these forums, so pleasing more than half the people with your difficulty is always a challenge.
But I think I've more or less got the hang of hard-core hard.