water still runs slowly when i switch off my aquatronic 4 shower?

Model Aquatronic 4 Shower. When I switch off the power on the shower unit it still runs cold water, (albeit a trickle) when it should stop within seconds. How can I repair this ? as i do not have a manual as I have recently moved into the house. Is it a electrical problem or has a valve stuck partially open due to scaling? If so how can I repair it?