I have a buttonhole!!!
Not in the fabric I want it in, but in a different area on the sacrificial jacket, but at least now I know I'm turning the right knobs and pressing the right buttons.
Though I've realised there's a ruddy great seam right where I want to put the buttonhole which I refuse to unpick, and which will probably mean there's no chance of the stitch working. So I'm going to think up an alternative approach, and probably wait till I've got a humper jumper.