Just a note to this. The seats do not use engine heat. They are equipped with a Peltier Element which can cool/heat. The fan blows through this element. Thus not a good indicator of coolant circulation.

Are the pipes going through the bulkhead to the heater matrix hot?

Edit:
Also, I assume the engine is at operating temperature when this issue occurs?

And you might also be interested in this thread, as it had a similar issue which was resolved. Apparently there is a change-over valve in the system that can block flow to the interior.
https://www.rangerovers.net/forum/6-range-...-heat.html 2007 L322 3.6 TDV8 Stornoway

At that other case there is no heat in the cabin.
Mine heats for sometime and gets cold if not set at 28.
At 28 heat is ok.
Manual heat input works for some time too then it gets cold. 2010 3.6tdv8 180k km.
sold 06 td6 160k
Sold 03 td6 260k

As you have the 3.6 TDV8, I will assume the car also has the auxiliary Fuel burning heater? This was standard on the TD-cars, yes?
As the case was in the linked thread, he got heat when the FBH is running. which it is when the car is started and outside temp is below 5°c. Could it be that you get heat only during the FBH operating period? Does the FBH assist when heat set to Maximum? 2007 L322 3.6 TDV8 Stornoway

Well I don't think mine is heating with FBH we are not yet below 5° yet and I don't know how long FBH works since mine starts to get cold after some time.
Today I did everything manual no lights on knob and it was better.
I must try that on highway too since it gets cold on highways and engine temp seems ok. 2010 3.6tdv8 180k km.
sold 06 td6 160k
Sold 03 td6 260k

Same thing happened.
It went colder as I got to highway.
It was 26 and 1st h eater speed (1 dot)selected.
No auto lights.
It should stay hot. 2010 3.6tdv8 180k km.
sold 06 td6 160k
Sold 03 td6 260k