Don't forget to put something on the floor near the radiator 'bleed valve' to catch the fluids that will escape once you start 'slowly' opening the valve ( turn anti-clockwise);you will hear the air coming out ,followed by the water/muck, which will run into the bowl you placed under the radiator. Screw the valve back tight ( not too tight,you might have to open it again and bleed a bit more), then clean up and luxuriate in the heat! Good luck.
If the top is warmer than the bottom, the fault is not gases in the radiator. Hence it cannot be rectified by bleeding.
Think about it - gases at the top mean no water in contact with the metal radiator surface at the top, ergo the radiator top is not as hot.
More likely to be unbalanced, as one person has suggested, or sludge causing a reduction in water flow.
Try turning all other rads off and run system for 30 mins.As bm says it may be sludge or a valve blockage.Is there a bypass valve in the system..it may need adjusting.