It was a cold Saturday morning in April 1988 when a van full of detectives arrived outside the North London home of Erwin van Haarlem. The self-employed art dealer, 44, lived alone in sleepy Friern Barnet, a smattering of brick homes beside the grim North Circular ring road.
The Dutchman's apartment building on Silver Birch Close had become the centre of an investigation led by the British intelligence agency MI5. It suspected that Van Haarlem - whom neighbours described as an "oddball" - was not in the art business at all, but a sinister foreign agent.
Inside, Van Haarlem was hunched over a radio in his kitchen. He was still wearing his pyjamas, but his hair was parted neatly to one side. He was tuned in, as he was every morning, to a mysterious "number station". In his earpiece, a female voice recited numbers in Czech, followed by the blip-bleep of Morse code.
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