low pressure warning light in Jade Trintion power shower?

hi - my Jade Trinton power shower keeps showing low pressure warning light. I checked the pipe feeding the shower unit, it is perfectly ok. The pressure is fine. I removed the shower and checked too. I opened the unit to see any obvious faults - none spotted. It does not even let cold water flow. I suspect it is the pressure gauge at the bottom of the shower. Can i replace it. I opened it so i know it can be. But where do i ge a new one and does anyone know what i can do to repair it.