The Goethe Exhibition at the Schism Kunsthalle gallery had been a hit. That Thursday the Frankfurt art gallery was open late and was busy but nobody noticed that shortly before closing time, two men slipped into a broom cupboard.

As occasional host to some of the world’s most valuable paintings, the gallery was heavily alarmed out of hours. But at 10.30 pm, the guard switched off the alarm in every room so he could make his evening patrol.

The two men in the cupboard knew this would happen. At precisely this time, they crept out and overpowered the guard. Then, they calmly unscrewed three paintings and carried them through a loading bay to a waiting van.

Two of those painting were Shade and Darkness and Light and Colour by J.M.W. Turner, on loan from Tate Britain. Together they were worth $44 million.

It was July 28, 1994. Charley Hill, head of the metropolitan Police’s Art and antiques squad, made his way to Frankfurt. He had asked that his German-speaking friend, Jurek Rocky’ Rokoszynski, also a Met detective and currently working undercover in Lubeck, north Germany be present.

The gallery where the painting had hung was now a melee of German and English police, government representatives, insurance agents and curators. Most attention was focused on a man called ‘Colin’ whom Hill had brought with him from London. He worked in a west London restaurant favoure by Greek and Serbian criminals and said he knew where the paintings were.

Colin was offering to facilitate their return – from the Swiss Chateau of a crime boss where he said they now were – for a fee of $92.000. Could he provide photographic evidence of the painting to back up his story? Of course, he said.

Then detective Sergeant Rokoszyski, a physically imposing 49-year-old, learned forward. “If you can’t show me photos, get this man with the paintings on your mobile. I want to speak to him.

Colin seemed unwilling. “I can’t. The battery on my phone’s gone flat.”

“Use mine,” Rocky offered.

“His number’s on my phone.”

Rocky snorted and stood up. “This is a conman. You’re wasting my time.”

He got back into his car and drove back to Lubeck.