Well, not a total disaster, but I think I've either killed them with kindness, or they were just diseased. Eric says no-one could have been more attentive to their needs, so it's not neglect. Ignorance maybe.
At the moment, I have 12 cocoons (although I know for certain that 2 of the caterpillars have died inside), another 3 or 4 spinning (but very sluggishly), and I've just put the other 13 in toilet roll inners, with some leaves, and told them to get on with it! The best I'm hoping for now is that some of them will hatch into moths, mate, and lay eggs for next year. In the meantime, I'll do a lot more investigating on the internet and see if I can find anything glaring that I've done wrong.
Next year will be better!!!