anyone any ideas please, the engine on my 300 tdi was rebuilt less than 2000 miles ago recieving new cylinder head etc, since the rebuild the engine has performed faultlessly, when on the move the temp goes up to around the half way mark and the heater blows nice and warm, however when stood still just ticking over the temp steadily drops and the heater blows colder and colder which is not good in this weather, please help, cheers