[wiizii]

Trying to change ballcock on hot water header tank have shut off valves under sink and in airing cupboard but still have water going into tank why?

[hkstroud]

Just guessing here but something tells me that;

You are not US;
You are talking about a small "point of use " water heater;
That you have turned off the valves between the heater and the faucet
And not the valve on the pipe bring water to the water heater.

[ballengerb1]

Ballcocks are inside a toilet tank, stop cocks are small shut off valves prior to a fixture. Usually you must shut down your main incoming water line to work on most water heaters.