A radiator that feels cool at the top and warm at the bottom has air in it, which must be 'bled' out. Each radiator has a bleed valve at one of its top corners, which is identifiable by the square-shaped shank in the centre. When you bought or had the radiators installed, you should have been given a bleed key. If this has been lost, then you can easily get a replacement from any DIY shop or hardware store.

Buy a plastic keyring for it, label it clearly and keep it in one place so you always know where to find it. Although it's not vital, switch off the circulating pump and the boiler. Place a jam jar or hold a cloth under the bleed screw and, using the key, turn the valve anti-clockwise about 1/4 turn. At first you should hear air hissing out - if you place one hand on the radiator you will feel the heat rising to fill the gap. Keep the key in the valve because as soon as the hissing stops, water will start to dribble out. Close the valve tightly with the key. Don't, whatever you do, open the valve more than you need to - or remove it completely - as dirty water will come spewing out of the radiator.