By header tank I assume you mean the expansion tank that is mounted on the off side of the engine bay. This should be filled only to the line about half way up it. It's job is to accommodate the coolant that would otherwise overflow from the system once it gets hot.
The system consists of the water pump, radiator, thermostat, expansion tank fitted with the pressure cap and heater matrix.
When you first start the engine the water is cold so the thermostat is closed and water circulates round engine and through the heater matrix. Once the engine reaches operating temp the thermostat opens and the water can now flow through the radiator. It flows from top to bottom. . As the water is hot it will have expanded and some will be pushed into the expansion tank and the level will rise. The pressure cap causes the pressure in the system to rise, water boils at a higher temp at a higher pressure. If there is an overpressure the cap will release the excess and some of the coolant. As the pressure decreases as the engine coolant so the cap will allow air back in but the level will drop.
The heater matrix is fed with water all the time irrespective of whether the heater is off or on. A diverter flap just sends all the incoming sir around the matrix to give cold air, it's more efficient than turning the water on/off.
So what could be wrong with your car.
A) The HDi engine do run cool.
the pressure cap is not doing its job
C) there is air in the heater matrix, there are bleed screws on the hoses that go to the heater matrix, these are in the engine compartment by the bulkhead.
D) the thermostat is not working properly
E) there is air in the system there is a blue bleed valve on the thermostat housing.
F) the water pump is worn however you get serious overheating if that was the case.
G) radiator fan is running all the time. The rad fan should run only when triggered by the heat sensor. This will usually be when the car is stationary as during movement the airflow through the radiator is enough to keep temp in correct range. However it also runs every time the sir on is switched on. This is because the air con condenser is mounted in front of the car rad next to the rad fan. It runs at slow speed during air con use. If while stationary the water over temps then the fan runs at high speed.
So there you are. List of what you need to check. I'd start by ensuring the rad fan is off when the engine is cold and the sir con is off..