How much water to clear an air lock?

Hi,

Ive moved into a flat and the hot water pipe seems to have an airlock. Water comes out of the pipe at first, then there is a lot of spluttering, the water comes back and then eventually stops.

I've forced water back up the hot water pipe in an attempt to clear it, but its not shifting. When I force the cold water up the hot water pipe, it moves up easily at first and then I can feel pressure building up, if I leave this go should it move it?

The landlord had a plumber in to change the element in the emersion the other day and I'm guessing its something to do with that. Is there an amount of water that could be too much to force back up the pipes? I don't want to do any damage given it can be fixed by the landlord and not cost me anything.

Cheers,

John