When I started researching my mother’s family, I had no idea what I would find, or how the process would affect me. Would I meet interesting relatives? Would I discover unpleasant secrets? Above all, would answering the question “Who do you think you are?” be a positive experience? Growing up, I was told a lot about my father’s side, the Alexanders, who in the 1930s escaped Nazi Germany and came to England. Their stories have always stayed with me and inspired me to write two books: Hanns and Rudolf about my uncle Hanns who tracked down and captured the commandant of Auschwitz, Rudolf Höss, and The House by the Lake about my great-grandfather’s weekend house outside Berlin, which bore witness to the 20th century.

History, and my personal connection to it, has always been important to me. I am strangely motivated to explore what happened before my time. When I hold artefacts from the past – whether it’s a crinkled birth certificate, a chipped but much-used cup or some well-thumbed love letter – I’m deeply moved. And so I return to my family’s stories, time and again.

All I knew about my mother’s family, the Salmons and Glucksteins, was that they were, it was said, the force behind J Lyons, which at one time was the largest catering company in the world. J Lyons as in Lyons Teashops, Lyons Corner Houses, Lyons Maid, Lyons Tea, the Nippy waitresses, Lyons Cakes, the Trocadero and the Wimpy hamburger chain. I grew up in the 1970s, just as the Lyons food empire was being broken up so, frustratingly, my own experience of the company is limited, but to many people in Britain and around the world the name still carries weight.

I do have one memory, though. When I was nine, my mother’s father Sam, or Popop as we called him, took me out for lunch at the Carvery in Marble Arch. I remember being awed by the plush red-velvet chairs, the perfect white linen and the sparkling chandeliers. Around us men in pleated chef’s hats and starched white jackets pushed carts piled high with food: oval platters of roast lamb, chicken and beef and, later, chocolate gâteaux, cheesecake, apple strudel and pear flan. “You can eat as much as you want,” my grandfather told me. He owned the restaurant, after all.

I began my research at the National Archives in Kew. There I discovered a customs record from August 1843, which was titled List of Aliens. This was evidence of my family’s arrival in London after fleeing antisemitic persecution in Prussia and the Netherlands. Next, I wrote to the General Register Office to obtain birth and death certificates. Through these documents I learned that one of my relatives died of a carbuncle. Another was illiterate and could not sign her marriage certificate. And then there was Lena and Barnett Salmon – my grandfather’s grandparents – who endured the death of six of their children. In one week they lost three kids to scarlet fever.

Some of the things I learned made me feel proud. In 1934, for instance, my great-grandfather, Isidore Salmon, told the owner of the Daily Mail and Daily Mirror, Viscount Rothermere, that if he did not withdraw support for the fascist leader Oswald Mosley, then J Lyons would stop advertising in his papers. For Isidore, this was not without risk. Ever since they had escaped Prussia, the family had kept their heads below the parapet. By taking a stand against Mosley, Isidore could be cast as unpatriotic. According to Mosley, Rothermere immediately withdrew his support. Starved of the oxygen of press publicity, support for the fascist movement dwindled.

There was also plenty for me to feel guilty about. My mother’s family’s wealth came originally from Salmon & Gluckstein Ltd, a tobacco business that they built in the mid-19th century. This meant that not only did they make money from selling cigarettes and cigars, but the tobacco they used came from Virginia, on land cultivated by slaves. This raised questions in my mind about responsibility and reparations. As it happens, I am familiar with the process of restitution and the challenges of intergenerational reconciliation. My father’s family owned a house by a lake near Berlin that was stolen from them when the Nazis came to power and which has only now, some 80 years later, been restored. This time the shoe was firmly on the other foot and it did not feel quite as comfortable.

To truly understand my mother’s family, I knew I had to get beyond the public archives. I had to gain access to their private letters, unpublished memoirs and photographs. But to see these precious artefacts, I had to convince my distant relatives to hand them over, people who I wouldn’t have recognised in the street.

I started making phone calls. One relative put me in touch with two others. “Have you spoken with… ?” I was asked and the connection was made. I dialled the number, introduced myself and said where I fitted into the family tree – “I’m the son of Belinda, the grandson of Sam and Wendy, the great-grandson of Isidore and Kitty” – and then, assuming an intimacy that was not there, asked about their lives, what the family meant to them, how it had made an impact.

Some of the conversations were over in five minutes, others went on for more than an hour. I spoke to one cousin in Wellington, another in Rio de Janeiro, a third in Paris. I travelled to people’s homes. I met them in cafés. I almost made one cousin miss a plane in Portland, Maine. “My wife’s good at this stuff,” he yelled down the line as he hurried to the boarding gate, “we’ll send our comments by email.” I kept a note of our conversations and compared people’s responses.

It turns out that many of my relatives on my mother’s side are still actively involved in food-related enterprises, for instance Alastair Salmon, who is head chef for the Lord Mayor of London, Lucinda Bruce-Gardyne who founded the gluten-free innovator Genius Foods, and Nigella Lawson, whose great-great grandfather was also Barnett Salmon. But speaking to my newfound kinfolk, I learned the legacy of the Salmons and Glucksteins was a mixed bag. To most, it evoked awe and profound nostalgia, memories of luxury and the crème de la crème. To some, particularly the women, it triggered anger, reminding them of how, in the company’s beginning, they were excluded, the founders barring women from working. Most agreed that the family was remarkable, although there was little consensus on why. Some shared childhood anecdotes, eating at the Carvery, free lunches at the Trocadero, while others spoke of common family traits such as sending badly cooked food back at restaurants, “Out of respect for the chef.”

Several were keen to forget, to sweep it all under the carpet and move on. Quite a few believed Lyons and all it stood for was yesterday’s news, with little of interest today. With these last views, I disagreed. For it is indisputable that the family revolutionised gender employment, high-street food retailing, army catering, food distribution (particularly fast food), product advertising… the list goes on. They were pioneers, democratising luxury and globalising taste.

More than this, the family’s story spoke of something bigger. Of a country built on the extraordinary efforts, cultural, scientific and economic, of immigrants. And it struck me that while other countries, such as the US, have historically honoured those arriving at their shores, through myths like the American Dream and Melting Pot, we in Britain have not. We have rags-to-riches stories – Arthur who draws a sword from a stone, Dick Whittington who comes to London barefoot and later becomes Lord Mayor – but no legend that celebrates the British Dream. Surely this is problematic.

Now that the research is complete, has answering the question “Who do you think you are?” been good for me? The answer is yes. Learning where I have come from, both the good and the bad, has grounded me. Given my life more architecture, more meaning. And, just perhaps, it has given me a little more courage to put my head above the parapet. Like my great-grandfather Isidore did when he bravely stood up against Oswald Mosley.

Legacy: One Family, a Cup of Tea and the Company that Took on the World by Thomas Harding (William Heinemann, £25). Buy it for £22 at guardianbookshop.com. Follow Thomas on Twitter @thomasharding