“I’m so sorry,” said a passer-by who appeared merely to touch Georgi Markov, a Bulgarian refugee, as he was walking along the Strand in London.

The passer-by in fact was Markov’s murderer. His “touch” was a slight stab into Markov’s leg with the deadly poisonous point of his umbrella. After his apology the killer hailed a taxi, and Markov, unconcerned, went on his way to make a broadcast in the World Service news bulletin to Bulgaria.

But by that evening of Thursday, September 7th, 1978, his leg was stiff and he felt feverish. Next day his wife took him to hospital and three days later he died.

In the post-mortem a minuscule hollowed-out pellet, made from an alloy of platinum and iridium and measuring just 1.53 mm in diameter, was found in his thigh. As tiny as it was, the pellet contained enough ricin, a derivative of castor oil seeds, to kill Markov. Although never proved, it was accepted that the killing was the work of the Bulgarian Communist secret police.