Loss occurred gradually with reduced pressure for a week or two followed by complete loss of pressure Friday evening.

When any hot tap is opened there is an initial flow of water which then dries up. If the tap is closed for several minutes and reopened there is another initial flow implying some slow flow of water into the system.

The header tank is full and the exit from the tank is clear.

We the disconnected the inflowing pipe to the stop tap immediately before the hot water tank there was a strong flow of water and good pressure at this point.

We have tried driving mains cold water through the system. The pipes to the taps fill but there is no flow up the water outlet of the header tank. Instead flow comes through the boiler vent pipe back into the header tank.

It has been suggested to us that the valve at the top of the hot water tank is one which prevents back flow and so we are not driving any water into the hot water tank to clear the air-block.

Is it worth trying to drain down the whole system? If so, how do you go about this?