I used to be quite a dab hand at using a rubber pipe attached to hot and cold taps to blow back air-lock to hot water tank. However, now that I have a combi boiler I wouldn't attempt this. Hope someone can help as I agree it's very annoying.
If you have a combi boiler you shouldn't get an air lock as both hot and cold are at mains presurel drive out air as they are at mains pressure. It usually occurs in gravity fedsystems.
There are a number of possible causes .. have a look here.
https://www.emergencyplumber.uk.com/plumbing/hot-water-airlock/