The only way now then is to try and force mains pressure water through it to push the air out
This is what I'd do..
Make sure all your radiator valves are open
then find the header tank for the heating side (usually the smaller of the two) and then identify the expansion pipe,, connect a hose to the expansion pipe and to a cold water tap and slowly turn the tap on.. this will force water through the system and it will start to fill the header tank through the outlet (any air should come out here also),,, allow it to do this for 5 mins or so - it'll overflow through the overflow pipe (that's expected) just don't allow it to overflow the sides of the tank
If you are unsure as to what you are doing then get a plumber
HTH