TV series set during WW1

Its time for another one of those threads where I try to identify a tv series that I saw back in the mists of time.This one was shown in the late 70s or early 80s and it might have been a kids series as the main character was a teenage boy.It was set I think in France or Belgium during the First World War and I think the main character may have been a spy.The episode that I remember had him held prisioner by this couple who were collaborating with the Germans and they might have been gypsies.They brought him out of a farmhouse to a car and the man,who was a big strong guy with a beard was carrying a Lewis machine gun like in the picture below over his shoulder.He fitted it then onto a mount on the back of the car and the woman was driving.Then a British fighter biplane was swooping over the car as it was driving and the big guy was shooting the machinegun at it as it passed over.After the plane passed over about three times it was shot down and crashed.The next bit was the boy was pulling the pilot from the burning wreckage of the plane.I'm sure it was a British series.Thats all I can remember,does it ring a bell with anyone?

Pic:Lewis gun of the type in the programme.