I need to change the ball valve of the tank in the loft. There is a stopcock on the pipe leading to the tank. The reason I need to change the ball valve is that even after bending the arm to the float the valve is still leaking and causing the water to go down the overflow at most inopportune times (ie at 3.00am and waking me up!). I have a couple of questions.... How do I know what size I need to buy 1/2 inch or 3/4 inch - the tank feeds the hot taps? Having looked on the internet for purchasing a ball valve there was a site which talked about valves for low or high pressure, how do I know which I need? How do I go about replacing the ball valve? Advise would be much appreciated!