Just to add to Robs suggestions...
With ther rad cap off, and with the car up to temperature, hold the throttle at around 2500 rpm for a few seconds. This will shift any stubborn air locks.
Be very careful though, as the temperature of the coolant rises, the pressure will also build. This will force the coolant out of the rad filler, if the cap is off. You'll see the coolant start to bubble, in the filler neck, as this starts to happen though. When the level starts to rise, get the rad cap back on quick !
Paul