How can I repair my Aquatronic 3 electric shower?

My Aquatronic 3 electric shower Model No 97800270 refuses to turn off the water coming through the shower head. The power lights work ok and the thermostats regulate the temperature ok. The water flow through the shower head seems the same as usual - hot and cold. The micro switch connected to the on off button is OK. but from then on I am lost. To turn the shower OFF I have to switch OFF the separate electric supply shower isolater switch and then turn OFF the water flow through the independent isolater inlet valve. The on/off button on the housing doesn't stop the flow as it should! Any ideas would be welcome.