First of all isolate the circulation pump and turn off the boiler, although that is not essential. Every radiator has a bleed valve on one of its top corners, identifiable by a square-section shank in the centre of the round blanking plug. You should the right a key to fit the shanks. Installers such as Bounds Green Plumbers have these. But if not, you can buy a key for bleeding radiators at any DIY shop or ironmonger’s. Use the key to turn the valve’s shank anticlockwise about a quarter of a turn. It shouldn’t be necessary to turn it further but have a small container handy to catch spurting water, in case you open the valve too far. You will probably also need some rags to mop up water that dribbles from the valve. Don’t try to speed up the process by opening the valve further than necessary to let the air out – that is likely to produce a deluge of water. You will hear a hissing sound as the air escapes. Keep the key on the shank of the valve; then when the hissing stops and the first dribble of water appears, close the valve tightly. If no water or air comes out when you attempt to bleed a radiator, check whether the feed and expansion tank in the loft is empty. If the tank is full of water, then the bleed valve is probably blocked with paint. Close the inlet and outlet valve at each end of the radiator, then remove the screw from the centre of the bleed valve. Clear the hole with a piece of wire, and reopen one of the radiator valves slightly to eject some water from the hole. Close the radiator valve again and refit the screw in the bleed valve. Open both radiator valves and test the bleed valve again. Professional help from a Bounds Green Plumber is always a recommended way to carry out such work if you are unsure and will also guarantee the work is done correctly.