If your heater has gone cold you are low on water in the top of the engine,dont rely on the header tank because its up to the mark.
Remove the bung in the radiator,remove the filler cap on the header tank, fill with coolant till it comes out of the radiator hole, replace bung,fill header tank just above the mark you can see in the filler hole,replace the filler cap,on top of the thermostate housing there is another bung (the same as the radiator)remove and fill with coolant,squeeze the top hose a few times to get any air to the top repeat fill and squeeze a few times till you cannot get any more coolant in,squeeze top hose and replace the top bung at the same time,turn the heater controls to the hottest setting and start the engine,it should bleed itself, check for leaks and take for a run till it's hot.
If you are loosing coolant slowly over a period of time get the "P" gasket changed as this allowes you to loose water very slowly.300tdi's over heat without it showing on the gauge,no water around the senser no hot reading.
I hope this helps,if i've forgotten something,some one else will fill the gaps.
