Every plumber should have an vacuum cleaner in their van. Airlock are primary found in gravity fed system where there is no pump on the system. Either way the solution is exactly the same for every system or installation. Drain the system down entirely open the bleed at the highest radiator and use a vacuum cleaner to create suction from the lowest point.
If you suspect that you might have an airlock in a specific radiator, then a rubber mallet is a simple trick i use to release airlock and trapped air bubbles. or isolate and completely drain just that single radiator and stick a vacuum on one end. Afterwards replace the radiator, reconnect and bleed.
To avoid airlocks in the first place think of how a bottle or a cup is filled up from the top down, with the top fully opened. Similar filling up your heating system with the bleed point on the highest radiators left open. and fill up your entire system from bleeding from the lowest point to the highest point. best to have an apprentice with you at the highest point with a bleed key.