cooling fan works fine?

and i have bled the cooling system several times to no avail,with the header tank cap on or off it still overheats,( the heater only blows cold air out),wether the engine is hot or cold,could it be the heater ? the heater rad is not blocked and the heater fan operates but only blows cold air out even with the engine glowing ?,i am at a loss mate.geoff.