As promised, here's the sum total of my work so far (that's vaguely presentable, at least). I apologise for the rubbish sprite and lack of animation but then I've never been an artist.
Doubtless you will see the odd little quirk here and there - occasionally the game will crash if you fall off the bottom of the screen; this is because the sprite is actually too tall and if you're at the bottom of the bitmap its Y coordinates will fall outside it (there's 16 extra pixels at the top and bottom of the bitmap to allow objects to scroll smoothly on and off screen but the player is 20 pixels high which is a bit silly).
Still no inertia or anything and the collision detection isn't perfect but you can find that out for yourselves. Also when you die the program keeps scrolling forever - that's something I need to tidy up also. And I've changed the scrolling slightly so that as the top platform scrolls off screen, a new one scrolls on from the bottom (which is how the Jaguar version works), but have neglected to change the source code notes accordingly.
Plenty of work to be done, still. In the meantime, I need some graphics, music and sound effects if anybody is interested in contributing. And if people can offer their own advice/method for sussing the controls and collision detection properly that would be handy too.