It may just be an airlock mate.
Remove the coolant header tank and run the car from cold until the coolant starts to boil, then stop engine immediately.
You should see bubbles coming up through the tank.
Massage the coolant all the hoses while you do this, especially two about 1 inch diameter, top right of the engine bay, these are the ones that pass to the heater matrix.
Also, rev the car a little to get the coolant pumping around.
Then wait for car to cool, top up coolant if needed, then replace cap.