The problem is, playing with levels (if I recall correctly) only reached the right colours at the point where it made the outlines horribly jagged.

There's got to be some way with linking layers, using layer masks or blending modes to make the high-pass image print onto a canvas, I'm sure of it, but I simply don't know Photoshop well enough.

I'll try and post the scan tomorrow (deleted the original from my computer once I'd cleaned it up, but I can still get it back), in case that helps.