How to remove an air lock in a radiator or central heating system. 

hydroheatplumbi...
I created this video to explain to people how to remove an air lock in a radiator or central heating system, it's very straight forward and doesn't necessarily need an engineer to visit.
Your first step is to turn off all the radiators on the central heating system, except for the radiator that you suspect has an air lock. You do this by turning the radiator valve or thermostatic valve clockwise.
If you have a hot water cylinder you will also need to turn the hot water off, so that the central heating pump can concentrate on just the radiators.
Turn your heating on, and the pump has no alternative than to force the water to the radiator that has an air lock. Once an air lock has been released you should hear a gurgling sound or a glugging sound. This indicates that the air lock has been released.
You will need to release the air from radiator using a vent key, vent locations can be found on either the left or the right hand side, but can also be hidden on the back of the radiator.
If the air lock from the radiator has been released you need to wait until both central heating pipes are hot, and relatively close in temperature.
Your final step is to turn on all the radiators on the central heating system, and check to see if they are all getting hot.
Hopefully you're all sorted now, and you have just saved yourself a call out charge.
I hope you like the video, thanks for watching.