Increasing the Water pressure on potterton puma 80!?

After bleeding my radiators as a few of them were cold at the top, the water pressure on my boiler has dropped into the red area! It only has about 0.5bar of pressure, and drops to 0 if my heating is switched down for a few hours. Before i bled them the water pressure was around 1 bar before the heating was called for! I have been told that there should be a black tap underneath but fail to find it! Could anyone help in locating the exact area for this as the tap may have come off or been removed!