OK, I'm admitting defeat with the Stewart platform idea. My 3D printed "heavy duty" servo arms aren't up to it (slipping on the servo shafts) and I'm spending more time on it than I wanted; to the point where it's blocking me from moving on and it's very annoying. I need to have a think, simplify the mechanism and get better servo arms.

Below is a brief video where you can just about see how the servo arms slip and that I haven't got the tilt rotating about the correct point. Crap. The shell of the head assembly would have sat over the top platform and flush with the body when in its lowest position.