How can I fix my Mira Sport 10.8kW shower?

I have a Mira Sport Electric Shower. There are three heat settings, Low Medium and High. It works fine on Low and Medium but the water temperature does not always increase when the High button is pressed. Removing the cover and operating the buttons, revealed that the relay/solenoid associated with the High setting does not always activate. What is the remedy?

Art Good
November 2009