Franz explained that he had been born in France in 1880, the only son of Jean Descartes, a wealthy diamond merchant. His family moved around Europe at frequent intervals and by the time he was eight years of age, the young Franz could speak excellent English, German and Spanish, alongside his native French. However, as a result of his father’s declining health, the family finally settled in a large house in the provincial French town of Albert during the summer of 1890. Franz loved the house, with its elegant blue façade and the line of topiary trees that stood in large pots along the front of the building. And he had fond memories of the countryside throughout the Picardie Region of Northern France.

Within six months his father died, leaving the family with some assets, but some even larger debts. Franz’ grief-stricken mother, Karin - a German by birth - could not understand how the family could be left in such a position given Jean’s lifetime of successful business dealings. But, in short, she was forced to accept the situation, selling the townhouse less than a year later to pay off their debts and moving with Franz back to her hometown of Mansell on the banks of Lake Constance in Southern Germany.

All of this was stressful enough to Franz, but on his sixteenth birthday he received a mysterious package from a firm of solicitors in France that Jean Descartes had always turned to for legal advice. On opening the package, he found that it contained a small key and a letter written to him by his father. Jean had written the letter on his deathbed, without the knowledge of Karin, and had arranged for it to be sent by the firm after his death. Urging Franz to ensure that the letter did not fall into anyone’s hands but his own, he went on to explain that over the years he had accumulated a fortune in diamonds, which he always planned to live off in his old age and to pass on to his family. However, his health had worked against him and so he found himself in the position where he had to think only of the family he would leave behind. But herein lay a problem.

Jean had known for some time that Karin Descartes had a lover, the 28-year-old Mayor of the town. At first, he had ignored their liaisons, hoping that the relationship would not develop into anything serious. He explained to the stunned Franz that their marriage had been loveless for a number of years, so he had always feared a situation like this. Karin had not been prepared to give up her lover and the relationship had become public knowledge throughout the town, much to Jean’s distress and contributing to his ill health. As a result, he determined that whilst Franz should inherit what was rightfully his, he would not leave more than an adequate amount to his widow. In any case, Jean knew that Karin’s rich family back in Mansell would never see her fall on hard times.

Jean Descartes had struggled to think of a way of preserving Franz’ birthright without involving Karin and avoiding a complex legal process. He felt certain that any legal resolution would be challenged by lawyers working for Karin’s family. This he could not risk . But he did, finally, engineer a solution. He explained that he had withdrawn from the security of numerous bank vaults, his full supply of diamonds and had placed them within a locked safe built into one of the interior walls of the French town house. This had been bricked over and the whole plan had been executed in secret when Karin and Franz had been away for a week in Paris. Franz now held the only key to that safe and Jean wished him every success and happiness in his life ahead.

Franz could barely take all of this in, as he read the letter in the drawing room of his new home in Mansell. Without his mother’s assistance he had no way of getting back to France and even if he could, had no idea how he could locate the safe and liberate its contents. And, to make matters worse, Karin Descartes herself died in a boating accident in 1897. In her written will, Franz learned that she had left all of her wealth to her relatives in Mansell, as she had it “...on good authority that my late husband has provided for our son, Franz, in some manner which he has not seen fit to share with me.” Franz felt betrayed.

Penniless and estranged from his mother’s family, Franz joined the Naval Reserve and began to train as an engineer in the Imperial shipyards in Kiel. Immersing himself in his work, he tried to forget about the diamonds and his parents. He enjoyed the work and was well regarded by his employers. In 1899, he married your mother, Nicole - a dark-haired, blue-eyed girl from Munich - and the pair moved to Hamburg. And, in April 1900, you were born to them, Heinrich.

Your father made it clear to me that you and your mother were always the primary focus of his life. But at intervals, he thought about the diamonds back in Albert and kept the small key to the safe within a specially fashioned locket around his neck. He even arranged for copies of the town’s local newspaper to be sent to him in Germany, feeling certain that if the diamonds were ever discovered he would be able to find out and put in a claim for them. But he heard nothing.

The years passed steadily, but with the outbreak of war, Franz was drafted into the Naval Airship Division of the German Fleet Command and became a Stoker Petty Officer on board a Zeppelin airship. Seeing less and less of his family, and risking his life during every air raid over England, he promised himself that when the war was over he would travel to France and reclaim the diamonds for you all to enjoy.

Such was Franz’ story, told to me that morning as he lay dying in an English wood, far from his family. He had told me the story because he needed to tell someone - anyone - before he died. “I am not sure why I have told you all of this, Peter,” he said, looking suddenly tired and weak, “but you are a good listener. I have only one further request of you, and that is that you get a message to my wife and son to say that I love them both and regret that I cannot be with them. Please tell Heinrich that his father was very proud of him.”

I was deeply moved by Franz’ words as the life began to drain from him. I had only known this man for less than an hour in the most surreal of circumstances, but I already knew that I felt closer to him than almost anyone I had ever met. “What about the diamonds?” I enquired, “...is there no way that Heinrich can claim them for himself?”

“I doubt it,” he replied quietly. “I have never told Nicole or Heinrich about the diamonds. You are the only person who knows about them besides me.”

“What about if I help? I could let Heinrich know the story. He could travel to France when he is older, find the townhouse and get the diamonds. Surely there is hope,” I said in desperation.

Franz looked at me wearily . “My friend, you forget that we are at war. Whilst I am French by birth, my son is German. Even if the war were to end soon, I cannot imagine that he would be welcomed, open-armed, by the people of Albert. They would hardly be placated by the knowledge that he was the son of Franz Descartes, who went off to live in Germany and became a Zeppelin raider. Those with long memories will know that Madame Descartes brought shame to the town by her scandalous affair with the Mayor. In any case, what right would he have to enter the house and break down the walls in search of the safe?”

In that moment I knew that I could not leave your father to die without hope. I told him that I would make contact with his family and pass on his final words. But I found myself going further, desperate to help this dying man - a stranger and an enemy of my country. “What if I find the diamonds and deliver Heinrich’s birthright to him?” I ventured .