Toussaint is a pioneer in the field – and visibly successful. His home is a luxurious seafront apartment in Monte Carlo, from which he commutes by helicopter. In two decades of hunting he has made millions from the fees he charges for his work – and countless enemies in the art world. Despite the fact that the museums and private collectors he targets are in possession of stolen goods, critics have branded him a merciless plunderer, motivated purely by money. One newspaper called him “the 50-per-cent man” because he reportedly demands half the value of a recovered work as his fee.

Toussaint has lost count of the dozens of works he has recovered for clients across the globe. Early in his career, he helped an elderly collector in East Germany to spirit works by Kandinsky and Klee to the West without the knowledge of the communist rulers. The money from the sales enabled the collector’s grandchildren to flee to the West. In 2001, Toussaint recovered a Klee watercolour, Deserted Square of an Exotic Town (whose value may be as much as £200,000), from the private Kiyomizu Sannenzka Museum in Tokyo. It was the first time a Japanese collector had returned a painting looted by the Nazis.

His greatest success was tracking six paintings by Kazimir Malevich, who died penniless in 1935 after falling foul of the Soviet authorities, to New York’s Museum of Modern Art on behalf of 31 descendants. In 1999 the museum paid an undisclosed sum in compensation, said to be $5m, and handed over to Malevich’s heirs a work called Suprematist Composition. They sold it at auction for $17m – a handsome percentage of which went to Toussaint.

He has now embarked on his most ambitious mission yet: a quest for an entire collection, a thousand or so paintings and drawings that were looted by Hermann Goering, the Third Reich’s second-in-command, from Jacques Goudstikker, a fabulously wealthy Dutch art dealer of Jewish origin who fled Amsterdam on the eve of the Nazi occupation. The collector’s heir has already won a landmark pledge from the Dutch government to return 202 paintings, including works by Filippo Lippi, Anthony Van Dyck and Salomon van Ruysdael hanging in various museums and galleries. The Goudstikker story features masked balls in a castle, the love of a beautiful Viennese opera soprano, an accidental death at sea, and a little black book listing alphabetically the dazzling treasures of an ill-fated collection: D for Donatello, G for Goya, R for Raphael, Rembrandt and Rubens, V for Van Gogh…

In May 1940, as the Netherlands awaited a much-rumoured Nazi invasion, the 42-year-old Jacques Goudstikker braced himself for the collapse of the flamboyant, carefree life he and his family had built up over two generations. The biggest art dealer in Amsterdam, he owned palatial premises on the Herengracht canal – two canals from the more modest home of Anne Frank – which stocked a thousand works, many of them masterpieces. The fashion then was for 19th-century landscapes and historical subjects, but Goudstikker persuaded museums to buy and show 16th-century Italian art and 17th-century Dutch art. A skilled merchant, he also published lavish catalogues with full-page photographs of a quality unrivalled by his competitors.

The rotund Goudstikker had a taste for the high life. Apart from the canal mansion on the Herengracht, he owned a villa on the seafront outside Amsterdam, and the sprawling medieval Nijenrode Castle 15 miles away on the River Vecht, to which he travelled either in a luxury car or in his private launch. An amateur chef, he loved throwing parties at the castle, with guests dressed up as 17th-century Viennese courtiers, both men and women sporting ornate wigs decorated with flowers. He created real-life tableaux with local girls, fish and game to reproduce works by Vermeer and other artists, which he then photographed.

Goudstikker invited Austrian opera singers to his Vienna-on-the-Vecht party to entertain his guests. One of the singers, the glamorous Desi Halban, was to become his wife. Fourteen years younger than him, she bore him a baby boy, Edward, born days before the Nazis invaded and their world collapsed in May 1940.

Halban obtained visas and tickets for an ocean crossing to America, while Goudstikker carried out an inventory of his 1,400-work collection, arranging for 20 paintings to be shipped to America ahead of them. “Very soon, the day will come when we won’t see all this any more,” he told Halban. But he kept delaying their departure.

On May 14, 1940, the couple were talking to acquaintances in an Amsterdam street when Halban looked up to see paratroopers dropping out of the sky. They decided to leave that day. As they neared the port, they were stopped by a Dutch soldier. But he’d recently seen Desi in concert and only asked, “Are you Miss Halban?” before waving them through. They boarded the last ship out, abandoning their limousine on the quay, the keys in the ignition.

Two nights later, as the ship sailed through the Channel, virtually all its lights off because of fears of air attack, Goudstikker told Halban he needed some fresh air. When he failed to reappear, Halban left the cabin clutching the baby and shouting for people to help her find him. The search party found Goudstikker’s body the next morning. It was lying in a hold; he had fallen through a hatch in the darkness and broken his neck. Halban buried her husband in Liverpool and continued her journey to America.

A few weeks after Goudstikker’s death, Reichsmarschall Goering climbed the steps of the gallery on the Herengracht canal. Threatening confiscation, he “bought” an estimated 779 paintings in a sham transaction typical of the forced sales engineered by the Nazis. In the months that followed, two of the late Goudstikker’s employees handed the gallery over to Alois Miedl, Goering’s henchman, receiving a big reward of 180,000 guilders each. Miedl, under the orders of Goering, also gained ownership of the collector’s remaining art works, as well as his homes and his trade name. If anyone in Amsterdam mourned Goudstikker’s passing, they kept very quiet about it. Nobody stopped Miedl from continuing to trade under Goudstikker’s name. Between 1940 and 1944, Miedl made a fortune trading 4,000 works, many of them sold to Nazis in Germany.

In 1945 an American intelligence unit found many of Goering’s purchases hidden in salt mines near Hitler’s Eagle’s Nest hideout. Halban tried to recover the paintings but, after lengthy negotiations with the Dutch government, which treated her as a collaborator because the gallery had continued to operate during the war, she managed to buy back only 165 works in 1952.