how do I refill my glow worm boiler or bleed it?

re-filled rads after installing additional rad. Bled all through - OK. Turned on pump - clanking and knocking around boiler and through pipework back to the hot water tank. Turned off. Air-lock in boiler? Can I bleed this too? Also when I turned on the stat to heat the system and start the boiler - apart from the noise - the heater did not spark - was this because there was no water within the boiler? or is this another problem?