Does anyone know how to fix this ?

[southcoastboile]

yes its a hard water area, not pumped, just tank fed. I dont think its an airlock, all other taps are fine, and water is constant out the shower. think it needs a new cartridge which will hopefully spin out and the new one will go in its place, by cartridge I took it as meaning you will get the whole brass bit that is shown in picture 2?

I am pretty sure the makers are villaroy and boch, I looked on there website and they sell lots of similar looking showers but none the same as this one. I sent them an email but have had no reply.