Then, I cut a stitch in the neck line, and dropped back the stitches, to do an extra cable crossing.
That was when I realised that, when I deliberately left off the final crossing I had actually moved the whole cable outward. At this point, I thought I was going to have to rip the sleeves and front. But instead I somehow recrossed it and picked up the rows I had dropped down.
I then knit the neckband on smaller needles, so it wouldn't look sloppy, and with fewer rows, because Leon thought that looked better. And then it was done! I'm really happy with it, and Leon likes it too.