Will She Be Staying In A Hotel With...
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No best answer has yet been selected by waynolunno. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.You don't say which overflow but I assume it is the main loft tank where the ball valve is not working. Either the ball has sunk or stuck, needs adjusting or the valve seat needs replacing. Lift the ball up to see if the water stops completely. Usually easier to replace the whole valve after turning off the water main.
If you have a combi boiler then you have expansion vessel or pressure valve problems.