I have recently installed a new radiator upstairs. I darined the heating system leaving water in the downstairs rads allowing me to cut and replumb in the new radiator to a new position.I have tried to refill the system with all upstairs radiator bleed valves open but I appear to have either a blockage or more likely an air pocket between the header tank which is full, and the boiler.
I have removed the pump and no water is getting to it yet the header tank remains full.

If I disconect and blow through the new radiator pipe air comes out of the other radiator bleed valves.

The system is a vented system with a hot water tank.

The boiler will kick-in the pump starts running and all valves open but no water comes through to fill the radiators - the header tank remains full.

After a short while the boiler will shut off and therefore I cannot get either hot water or heating.

Any help gratefully received.

What is the pressure reading on your pressure gauge? You may have a bad pressure reducing valve, in which case, no water is being fed into boiler to compensate for air being bled out. IF boiler has no, or low pressure, the pump will not circulate the water. If this is a hot water boiler system, you should have 12psi at gauge, give or take, no less than 10, no more than mid twentys(when hot, say 180 to 190).