You can get the header tank looking like new. Cold water with some washing up power and some rice, lid on and shake. The rice will hit against the walls to help remove the residue or small ball bearings.
Then a cloth wrapped on a piece of stiff wire will allow you to remove the grot from the hard to reach places like the underside of the tank.
Then a cloth wrapped on a piece of stiff wire will allow you to remove the grot from the hard to reach places like the underside of the tank.