With a quiet Saturday morning on the cards, I decided to strip the engine completely and see if I'd done anything silly like leave out the crank sealing ring.
After sucking and blowing on the port which supplies the vacuum signal to the regulator, I found it's also ported to the intake side of the crank web via the crank pin and the holes in the con rod big end which line up with that port at different positions depending which way around you have the conrod. The old conrod had two big punch marks in the big end bush for orientation, the new rod has a tiny radius that really isn't noticeable so you don't need two guesses which way around I had the conrod on.
So with a new rear bearing fitted, conrod flipped, cam timing double checked, and engine back together with a stroke of luck I can get out to the filed some time this afternoon....
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My YS seems to dress slightly to the left as well..