Don’t worry about this, I have come to a resolution.
I have realised either I am doing something wrong or the mobile renderer just cant accept 2 different textured materials on the same skeletal mesh.
For now, my workaround is just to have only 1 material for the whole of my skeletal mesh (only 1 material element). This 1 material is masked in areas for transparency.
This is quite a nice optimised solution anyway.
Note how my material is masked, not translucent! Translucent materials at the moment seem to have Z ordering issues even on the desktop renderer!