Any other symptoms?
Doesn't sound like a head gasket - I'd agree with the airlock diagnosis. They can be a bugger to burp.
From Landyzone:
This is the procedure from the LR manual, I figure they spent some time getting it right.1. Remove bleed screw from heater return hose.2. Slowly fill the system through the coolant expansion tank until a steady flow of coolant is emitted from the bleed hole.3. Fit and close bleed screw in heater return hose.4. Continue to fill system through the expansion tank until coolant reaches to the base of the expansion tank bleed hose.5. Fit expansion tank filler cap.6. Start and run engine at idle speed for 5 minutes.CAUTION: DO NOT operate air conditioning7. Switch off engine.8. Squeeze bottom hose several times to remove any trapped air.9. Start engine and run at speeds varying between idle speed and 3500 rev/min for 7 minutes.10. Switch off engine and allow to cool.11. Remove expansion tank filler cap and add coolant until it reaches the 'MAX' mark on the expansion tank.12. Fit expansion tank filler cap.13. Start and run engine until normal operating temperature is reached.CAUTION: The bottom hose must be warm/hot, this will ensure the thermostat is fully open.14. Ensure warm air is available at vents.NOTE: If warm air is not available, an air-lock may be present in heater matrix. If necessary, allow engine to cool, remove expansion tank filler cap and repeat bleed operation at heater hose15. Switch off engine and allow to cool.16. Check for leaks and top-up coolant to 'MAX' mark on expansion tank17. Fit underbelly panel.18. Fit engine acoustic cover.