It all points to the matrix don’t u think, I understand stand the diesel engine is a cooler running than say the petrol engine, so with your coolant and thermostat change the engine temperature is now normal and as Tdis are.
If not, then although the matrix is a size that there’s now not enough cores.
Your issues reminds me of my RR which had a pathetic heat out from the heater in winter the main dealer checked it out supposedly and said it was so i had to live with for 10 years, the D1 is a lot better, i turn the output temp down after a mile or two in mid winter.
If you have a gurgling sound from the matrix, that is one sign of air in the matrix, lack of sufficient heat output is another, the matrix doesn’t have the means to bleed that air out, so drain and refill the system slowly, so the coolant rises slowly pushing air out, its a bit involved with the tdi I understand... the placing the vehicle higher up at the front when filling has also been mentioned in the past, which would bring trapped air into the top of the rad.
Any air will then travel to the reservoir which fitted has a valve to self vent, unlike the days past when the engine had to brought up to temp with reservoir cap off after replacing coolant.
