Solution: The radiator needs to be ‘bled’, as air has entered the central heating system and become trapped. The air will rise to the top of the radiator, forming a pocket that will stop the hot water from reaching that part. To effectively bleed the radiator, firstly turn off the heating system, in order that no more air is drawn in. Now, armed with a cloth beneath, use a radiator key (or screwdriver if necessary) to slacken the air bleed valve (located at one end toward the top of the radiator). Turn the bleed key a half turn anti-clockwise, which will release the air (there will be a hissing sound). As soon as water (which can be both hot and dirty, so use the cloth) begins to flow, close the air bleed valve by turning it a half turn clockwise. Do not over tighten the bleed valve.
Note that radiators should not require frequent bleeding. If they do, there is an underlying problem with air entering the system, and this should be attended to by an engineer.