Slums often have the nicest names, so don’t be beguiled by Blossom Alley, described by a detective in the 1920s as Portsmouth’s worst residential street. Here in the blood-soaked bed of a tenement building on Sunday, January 27th, 1923, Mary Pelham, a 37-year-old prostitute, was found with head injuries and a scarf around her neck with which her killer had tried to strangle her.

Was he a sailor or a civilian? To determine whether or not it was a serviceman the Royal Navy arranged an identity parade of 3,500 sailors, surely the biggest identity parade ever held. It was reviewed by a woman who saw Mary with a sailor just before she died. The woman picked out one man, but he was able to account for his whereabouts that night.

Blossom Alley has long since gone, and so too in all probability has Mary’s unknown murderer.