If your radiator is not giving off its usual heat and it is hot at the bottom and cold at the top, you need to bleed it. The process to bleed a radiator combi boiler is fairly easy and can be done in about 20 minutes. You will also need to have access to the boiler to do this job properly.
Turn off your heating system at the main switch. Your main switch is usually located near the boiler and will be clearly marked.
Place the bleed key in the bleed valve located at the top of the radiator combi. The bleed valve will either be a small brass screw recessed in the edge of the radiator, or it will be a small screw set in a knob on the side. The screw will either be a regular slotted type or an allen screw.
Cover the bleed key with a rag and place a small empty bowl on the floor below it. This will prevent you from being burnt by the water.
Turn the bleed key counterclockwise about a 6 mm (1/4 inch) of a turn. You will hear air "hissing" out. Wait until the hissing sound has stopped and a small amount of water has begun to come out of the valve. Turn the key clockwise to close the valve.
Go to the boiler and visually check the bar gauge on the top. If it reads less than 1 bar, add water to the boiler until it shows a complete bar on the gauge.
If you have lost your bleed key, a flat head screwdriver or small needle-nose pliers will work just as well.
If bleeding your radiator combi boiler does not fix the problem, there may be a more serious malfunction in the system and a professional repairman is required.