Update time I guess.
Heat treated the yew as slowly, carefully and deeply as I could. Took about 45 minutes for each limb, no cracks, checks or scorch marks when I was done.
It came off the former perfecty - dead straight. Tillered it slowly back to 28" without any significant loss of draw-weight and....
All the stringfollow has come back! It's back to about identical to where it was before the heat treatment (bearing in mind this has never been shot yet, so will take far more as it's used!)
I'm totally fed up with it now, won't be doing any heating and will just let it be a big chunky slow thing. Not pleased!
It seems that most of the stringfollow/deflex whatever is focused around an area where the propellor twist and lateral dogleg/bend thing come together and the sapwood gets bored and goes for a wander around one side of the bow. It's just above the arrow pass in the upper limb. I've taken a pic to show how it looks after tillering, but it doesn't really show the deflex properly. The piece of wood next to it is from the same branch so it's hardly surprising that the bow took some stringfollow in the middle, really...
It's clear in this pic where the problem is, but heat treating doesn't touch it, it just pulls straight out again.