Why won\\\'t my header tank drain?

[ordinary_jo]

I'm sure this is really straightforward, but I'm missing something!

I need to drain my header tank in the loft. There is all kinds of unsavoury matter in there at the moment. So, mains turned off (at the inlets under the kitchen sink) and taps turned on cold. But, the tank remains full with water coming out the taps. There is no central heating in our house, just cold water, hot water from an immersion heater, and this one header tank in the roof. I didn't try tying up the ball valve, but I will give that a go.

What else am I missing?

 

[JPC]

uhhh ?

the hot water commes from cistern. make sure have turned off the mains.
run the hot taps and check the cistern is not re-filling