Emma told him of the rules, ‘You must not photograph anything,’ she said,‘but you may make notes, only with pencil and paper we supply.’

She brought him a file marked ‘Strictly confidential’ and reminded him that the information must not be used for any media or publishing purposes. He was asked to sign a declaration to that effect. Emma then led him to a reading room. He noted cameras.

Cavalier spent the best part of the next three days reading Serov’s translated diaries and taking notes. On ten occasions he asked Emma for clarification of the original Russian, which was also given to him in a folder.

The entries were clear about the third Baron Bleulance. He had worked extensively for the French Underground and British Intelligence during the war, when he had also sent valuable information to the KGB via his good, French-speaking Francophile English friend, Sir Anthony Blunt, the ‘Fourth Man’ in the Russian-controlled Cambridge University spy ring.

The third baron had died thirty years ago but his son, the fourth Baron ( Jacques) Bleulance, was alive and well and living part of the time in his chateau at Berry, two hundred and seventy kilometres due south of Paris. Some cursory research in London discovered that Jacques hated his father, which meant he just might open up a little about him.

A Sunday Times business journalist told Cavalier: ‘The third baron in the mid-1970s eased into Bleulance Credit Internationale, pushed his son, the fourth baron, out, and replaced him with his son’s French cousin. This riled the fourth baron. He set up his own Paris-based merchant banking operation, which has been a monumental success. In the last decade he has gobbled up as much of the world’s energy operations as possible, especially oil.’

Cavalier, trying a long shot, found the fourth baron’s phone number. Using his guise as Dr Edwin Harris, Cavalier rang him on the pretence that he wished to write a feature on his art acquisitions for the British magazine Food, Art and Antiquity.

It helped that Cavalier spoke fluent French and had more than a passing interest in many forms of art from paintings to porcelain. Bleulance agreed to the interview. Cavalier then contacted the magazine, which was only too happy to accept his freelance piece on someone, the editor claimed, he had been trying to meet for twenty years.

‘You do realise that the fourth baron has a twenty-five per cent stake in us?’ the editor asked.

‘Of course,’ Cavalier said, although he’d had no idea.

‘Just be kind to him, please,’ the editor said.

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