That's very unusual in the primary circuit DT (28mm pipework from Aga to immersion cylinder).
Have you checked the header tank in the roof? If, for some reason, it ran out of water, then air would be introduced.
The plumber might undo one or both the connections to the Aga, and let them run to clear the airlock.
Annoyingly... these things often sort themselves out with a lot of gurgling in the system, but you can't rely on it.
I'd first look to see how the air got in there.