I have a receipt with my car that shows that the car had a new heater matrix a few months ago, the heating was working but if it was put on full and hot the engine temperature would drop and the heating would go luke warm.
My radiator was then replaced with a few pipes, and as part of it as a precaution I had the thermostat changed, however when the thermostat housing was taken apart, there was no thermostat inside. So the new one was fitted, the cooling system was flushed without the engine running and the mechanic went off home.
I left the Discovery on tickover for a while, the temperature gauge rose but when I put the heating on, it felt like Ice Cold Air Conditioning was on. I don't have air con in my Discovery.
Could this be an air lock?