So, I picked up the second sleeve this morning, with every intention of getting well along with it today. Around mid-morning, I went to my knitting bag for another ball of yarn, and as I took it out, two plastic pockets for circs fell out too. I keep the needles that are needed for a project, with that project whilst it is in progress, you see.
Anyway. I picked up the two little Addi packets and went to put them back - and stopped. The 3.75mm pack was empty. The 4mm pack had a circ in it.
I got out my needle sizer, hoping against hope that I'd put the 3.75mm one back in the wrong packet after I finished the ribbing - but no such luck.
I've been knitting this sleeve on the wrong sized needles.