Although she was married to a fishmonger, Kate Jackson lived the life of Riley at her bungalow in Limeslade, near Swansea. In 1929, when £4 a week was a working man’s salary, she would spend several pounds on flowers for table decoration in a single day. Her clothes bill was sky-high and she would dispense £1 notes as tips with regal generosity. If any extravagant whim took her, like hiring a car or a motor-launch for the day, she gratified it instantly.

And the cash? It came to her by post regularly every Wednesday – sometimes “a whole bundle of notes,” sometimes only two or three pounds.

“I’m a novelist and a journalist,” she would explain to friends. None of them, not even her husband, had any idea of the real truth. She was in fact an unusually successful blackmailer.

At a court hearing several years earlier, when a man was charged with misappropriation of union funds, it was said that he had handed over a large sum of money to a woman referred to as Madame X. The woman’s name was suppressed at police request in the hope that some restitution would be made by her in return. That was wildly optimistic, for Madame X was Mrs. Kate Jackson.

Her husband Thomas Jackson believed all his wife told him, and accepted the idea that she was a wealthy woman who did a little journalism and so on just for fun.

She was undoubtedly a mystery woman. Objecting to the name of her husband as being too ordinary, she persuaded him to be married in the name of “Captain Ingram.” Occasionally she let it be known that she was the daughter of the Duke of Abercorn. In fact, she was the daughter of a farm labourer named Atkinson.

On Sunday, February 10th, 1929, Kate Jackson went to the cinema with her next-door neighbour. When they came home each went to their own house. Almost immediately the neighbour heard a scream from Kate’s house. She rushed round to the Jacksons’ and saw Kate lying outside her back door. Her husband Thomas, who had just come down from his bedroom, was bending over her.

Kate had been hit violently over the head. She was taken to hospital where, according to a doctor, Thomas Jackson said, “I have been married to her for ten years, and I still don’t know who she is.” Six days later Kate died, and a fortnight after that her husband was charged with her murder.

The evidence was flimsy almost beyond belief, yet the judge’s summing-up was heavily biased against Jackson. As to whether a lurking stranger could have killed Kate the judge said: “There is no evidence of any secret enemy, that is merely surmise or possibility.”

The jury thought differently. They probably figured that there was every possibility of a secret enemy – like, for instance, the man Kate was blackmailing. They found her husband not guilty. For them, not only was the life of Madame X a mystery, so was her death, and so was the name of the man who murdered her.