So my third visit into the dealer for this issue . They replaced the blades again and already replaced the arm, but this time they also polished the glass with a compound to smooth it out. They said sometimes as part of manufacture process the screen is not 100% smooth and the polishing process should sort it. They tested after for quite a while in their wash bay with plain water and the problem was gone. Apparently they have seen it happen before.
The glass seems clearer now, but the proof will be driving in the rain which I haven't done as yet, So I'll see if it lasts.
Hopefully this time it is fixed