Mira electric shower won't stop

I have a 3 year old Mira Sport 8.5 Kw electric shower. After the water starts flowing and I turn the control knob back to the off setting the water continues to flow (cold) for up to half an hour. even with the isolator tuned off.
I opened it up and found that if I tapped the first black camber after the water inlet a pair of white levers slowly moved on the side and the water was shut off. Seems some mechanical valve is sticking. Is it worth dismantling the valve or just buying a new one? Is it serviceable? Has any one taken one to pieces?
David
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Try turning the water down at the mains isolator valve before you start to dismantle anything. If the flow valve in the shower is being held open by a high water flow, then turning it down a bit will allow it to close easier.
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