My daughter has a Gainsborough electric shower, model 8580 (I think). It's only putting out cold water. The electrics supply to the shower is o/k. When I took the cover off, there's a small plunger connected to a spring. When this plunger is pressed down manually the water is hot when the "on" button is pressed, but when you press the button to switch off the shower, the water is cold next time it's switched on. Is this plunger a thermostat control ? or could it be a water pressure problem ?