have you tried turning the heating on, this will run the pump, and switching the rads off at the rad valves?
Krisn - many apologies! I have just re-read your OP more carefully. Obviously from a drain down and yes - I did meant the F and E tank! diymostthings
Tom the pump and rads are on the other circuit from the boiler it's the gravity circuit that has the problem. That's why I think he needs to force the water thro the gravity circuit with a pumped supply from a power flusher or maybe a mains pressure hose arrangement.
Hey up Krisn, I,ll be air. If youve got any balls and your capable.................pull the phial out of the thermostat pocket, Get a good heat up and blow the bugger out. Turn boiler off and replace Phial, repeat if neccessary. Or get a Plumber in?
Air in return pipe. Make sure system as full as can be. Turn the pump around. Run heating 15 mins or till overflow kicks in. Put pump back and run again with rads open. If this does not work. Get a heating engineer to flush the damn thing out or convert to fully pumped then flush it. Dont spend your time thinking you will save time and money based on some comments on here. Good luck
Can you loosen a nut on the cylinder flow n returns to air it out? That's assumin you don't have an air vent