You have got a serious problem. The reason the ball cock is running water is because the the plastic header tank is bending/flexing due to the hot water in the header tank.
The only way hot water can get into the header tank is is up the expansion pipe. this means you have got water boiling in your system.
If you are using an immersion heater, the thermostat probe has failed and is not switching off.
If you are using a central heating boiler, it too has failed and is not switching off the boiler at the desired temp.
You MUST switch off the faulty appliance. I dont wish to be alarmist but the last time someone's plastic header tank ended up being full of hot water it burst and poured the hot water straight through the ceiling into a sleeping childs bedroom.
DON'T DELAY--please
Cledlyn
August 2008