Bet £100 its Definately an airlock! Best way is to jack the car up (or park on a REALLY steep hill) so the Rad is higher than the black rubber hoses above the rear of the spark plug cover, take the rad cap off and massage all coolant pipes while the engine is running, also make sure your heater is set to full whack.
Do this for about 20 mins then take it for a run (put Rad cap on again) to get the water flowing, let it cool down and check it, repeat as neccessary.
The problem is that the air gets trapped in the above mentioned black rubber hoses at the rear of the spark plug cover as they are the highest point in the coolant system (air is lighter than water: duh)
hope this helps:D