I played about with some mild variable damping across the line scan coils. It consists of a 5k wire wound pot in series with a .001uf capacitor. With the pot set about mid way there is an improvement that can be seen in the last picture.
You are quite right Andy. I have a number of B18T series and the line linearity is 100% on all of them. My guess is that the primary has too few turns to load the PL38 correctly.
I think this is about as good as it is going to get with this transformer. The EHT is 6kv and remains so all day.
In spite of the non linearity, Fred and Ginger still look pretty cool to me.
Pictures taken under different workshop lighting. The picture is very sharp producing strobing effects.
Thanks again for your comments. John.