Thanks, that does make sense. The sequence with the pockets/pinch on the arete is more or less what I was trying. I couldn't work out a body position enabling me to bump my left hand up, though I did manage to get into the undercuts if I started from the pinch. It was a little damp so perhaps that may make a difference, and there are a lot of permutations for feet there that I haven't tried.
I can't see how you would get into the undercuts without the arete, and while I have no real idea of how hard the moves on an 8a are supposed to feel, I can't imagine how I would do it like that, whereas with the arete I can at least see how it would be done.
Going up and right after you get the twin undercuts seemed the logical thing to do to me (at least that was the way the chalk went), I couldn't see how it would be possible to continue up the arete but hey ho.