Feeling I have been dumb. Just noticed you said "header tank" as in the tank that keeps the central heating system topped up.
I think it might even be called the "expansion tank." That makes more sense as indeed they do have a ball valve and an outlet that pours into the tank, which (sticking my neck out here) comes from the system when the water expands on being heated by the boiler and has to go somewhere rather than blowing up! Ours is a big pipe that goes up the outside of the tank then curves over to let the water fall into the tank.
But again I may be wrong and your ordinary cold water feed tank that is huge and sits in the roof as originally organised Lord Congreve! Is the one you are talking about.
"
In 1809, Congreve rockets were used by the Royal Navy to set fire to Amsterdam. They did a very effective job as there were no water sources available to put out roof fires. The UK government realised the vulnerability of London, and decreed that all buildings should have a large water tank in the roof."
@brianp38dse or
@gstuart may be able to help.
Glad you are, at least at the moment, sorted.
(Don't forget to NOT insulate under the tanks!!!!)