The story is set during the Second world war and deals with a group of children living in the North East of England in a town called Garmouth regularly suffering from bomb attacks by the German forces. One of the children raids a German aircraft that crashed in the area and takes a fully operational machine gun, intending to set up their own fortress. They fire the gun at a German plane, missing it but forcing it to crash-land; the gang befriend the pilot, not knowing that they had caused him to crash.
I hope that is what you were talking about.