Once upon a time, that time being approximately 1915, there were two little aerodromes in a big, scary world. In between them ran a teeny road, called Plough Lane, a hark back to even older times. When the aerodromes linked into one Croydon Airport, the lane was still open to public traffic: halted by a man with a red flag if a plane was due. Somewhere in the 1920s a gate was installed: times were getting less quaint, more pragmatic.
Croydon was the main London Airport and a pioneer of air traffic control. It is not exactly clear (from first Google search) when Frederick Stanley Mockford (1897 - 1962) became the senior radio officer, nor exactly what event prompted nor what particular date it happened but it does seem reasonable that he was asked to think of a word that would convey an emergency situation, easily understood by all pilots and ground staff. It is likewise reasonable and feasible, since much of the early days air traffic was between Croydon and Le Bourget Airport (Paris, France) that he used a French phrase for inspiration: "venez m'aider" translates as "come help me." Pragmatic, but from a romantic source.
The phrase was possibly most put to use on the 15 August, 1940, when Croydon Airport was the first London target struck during the Battle of Britain. Four airmen, one officer and one telephonist were immediate casualties. Nearby several factories were gutted, including the Bourjois perfume plant: there were 62 civilian casualties, 192 injured. One imagines the surreal horror: the sudden change of landscape, of lives, and the odd thought of perfumery in that devastation. Eight of the bombers were shot down too.
The airport, in times less dramatic, with upstarts like Gatwick popping up, was eased to retirement, closed in 1959.
The cut ends of Plough Lane were never reunited, the land became a park and a residential estate with roads named after famous aviators.
And, to end on a happy note, with no Mayday required: let us cheer for Amy Johnson, who was the first woman to fly from Croydon to Australia: and she came back again, in grand old 1931.