If it is a sealed , pressurised system (and no small feed-and-expansion tank in the loft to replenish the water), it is unlikely to be air but hydrogen. This is the byproduct of corrosion between the water in the system and the inside of the steel radiators. You need to put some corrosion inhibitor in the system.
If it is an open vented type (with the little tank in the loft), it is probably air and it is getting in through this loft tank. Is the level of the tank correctly controlled by the ball valve? Does the water level in this tank seem to rise and fall quite a bit whilst the system starts up?