First check your hot water cylinder to see if there is a manual bleed valve just above the uppermost boiler circulating pipe. Most cylinders auto vent these days but you may have an old or converted one.
If there isn't one, switch the CH pump off and close all rad valves upstairs. Now turn your boiler up to it's max on the boiler thermostat. This should force the hot water round the cylinder circuit and push the air lock out.
You can check your progress by checking the temperature of the top circ. pipe at the cylinder.
Don't forget to reset the thermostat afterwards.
Cledlyn
September 2008