You have described your system well, if the stop cock you mention below the header tank has a red wheel type head, this is whats known in the trade as a gate vale, when turned off
(in the clockwise direction ) they stop the flow of water to the hot tank allowing work to be carried out on the down stream pipe work. WARNING!! if the system is old and the valve is stiff due to lack of use, these are prone to snapping in the off position and then require replacement, suggest you just turn off at the main stop cock, open the hot tap until it runs dry and then carry out the repair.
wally ladd
November 2007