The fabric is something synthetic; it was 5 euros a metre on sale and is incredibly drapey, toasty and warm. And WOOHOO IT FITS! I don't know why I got my knickers so much in a twist about this; perhaps I'm just very unused to the way knits behave, and how one kind of fabric will sew up and hang totally different from another. What I ended up with is a Medium with a no-dart FBA (2 cm added) and a narrow back adjustment (2 cm removed). This was in fact one of my first goes at adjusting it to fit, but in my misguided frustration I had chucked the modified pattern pieces in the bin. Two sets of changes later, I followed the instincts I should have listened to the first time, and dug them out and ironed them.
Tell me you've never done it.
I'm just glad I didn't decide I was beat by this pattern after all - because it's lovely :-)