I'm not a heating engineer but, first of all, which header tank do you mean, the central heating header or the hot water cylinder header ? I don't think the central heating header tank water level should drop so much that you can see a tide mark where the level normally is, the ball cock valve...
After bleeding any air from the pump, put the blade of a screwdriver onto the pump body (whilst its running) and press your ear against the handle ( like a stethoscope) if the pump sounds very noisy, chances are its knackered. A new pump might solve your problem.
Is the hot tap water getting...
Suppose you have checked all rads for air locks, its worth checking to see if your pump is air locked ( usually a big chrome screw flush with the middle of the pump body) unscrew it a bit and see if you release any air.
Good luck :wink: