I was wrong in my previous update. I saw that Bella had rotated and jumped to the conclusion that it would carry on rotating. Oops. This morning I found Bella pointing back into the house again:-
It seems that Bella still rotates a third of a turn clockwise each day then resets at night, as per any Northern plant (plants from below the Equator rotate anti-clockwise). This pattern of rotation hasn't changed, only because the curve of the plant has been deliberately turned toward the pole, Bella now rotates to point away from Sol instead of towards Sol. Funny old world. So I haven't yet established if phototropism will beat heliotropism in getting a bramble to point at the Sun, all I've done thus far is to mess about with Bella's heliotropic behaviour.
Still, it's interesting to know.