Bravo, WD40 Degreaser!
I gave the cassette off the Cannondale a quick seeing to with GT85 and an old toothbrush to get the thick of the gunk off. It did do that, but it was still pretty black. This morning I felt guilty about not doing the job properly so I disassembled the cassette completely and sprayed all the bits with WD40 Degreaser. Left it on 5 minutes and the remaining grime just rubbed straight off.
Some of the Shimano branding also came off the locking ring, but I’m not too bothered about that. About half of the bristles fell out of the old toothbrush I was using, as well. Life is hard.
Gave all the cogs a good wash with detergent and dried everything carefully. Will put them on the new wheel this afternoon.
If you’ve never disassembled a Shimano cassette before, no need to worry about putting it back together. The splines on the freehub are designed so that you can only fit the cogs on the right way round, and unless you’re blind you won’t put them back in the wrong order.