[D.R.]

Did you replace the coolant recently?

From refill you have to open the bleed valve on the coolant elbow running from back of engine to lower Rad hose (petrol engines) not sure about the set-up on the diesels.

Lower coolant valve is 8mm bolt. Its located on the passenger side you will need to remove air filter housing to get at it.
to refill:
Open cap, loosen 8mm bolt and heater hose bleed screw.
bleed until coolant runs without air bubbles from lower valve, tighten the 8mm bolt.

Keep expansion tank topped up as a low level can draw more air into the system.

Eventually coolant will flow from valve on upper heater. Again bleed until all air bubbles are removed.

Start engine, run for few minutes, bleed again from heater matrix hose only.

Repeat until all air is removed.

 

[D.R.]

Yeah just open the bleed screw on the heater matrix hose. Turn the heater dial to full heat, stretch the hose so the valve is at the highest point, this will cause the air pockets to rise to highest point, then open and bleed.

Run the engine for few minutes then repeat.

Have you checked the oil and coolant for signs of gunge, sometimes hgf can cause this leading to air trapped in system. But dont panic just yet thre are other things to rule out before hand.

More than likely it was just 'contraction' of the coolant in the cooler weather that caused the air pockets, air gets drawn into the system due to low levels in the expansion reservior