- You may be looking for his grandson.
Alistair Conall Hamish Lethbridge-Stewart was a member of the Lethbridge-Stewart family. His father was the Honourable Hamish Lethbridge-Stewart, and his brother Archibald Hamish Lethbridge-Stewart. He was, ostensibly, the adoptive grandfather of Brigadier Alistair Gordon Lethbridge-Stewart, who was named after him. He was married to Lillian Lethbridge-Stewart (née McDougal), and they had two sons, Matthew Lethbridge-Stewart, and Gordon Conall Lethbridge-Stewart, who was born in 1902. He lived in Carmunnock, Lanarkshire in Scotland until around 1925, with his wife and sons. (PROSE: The Wages of Sin, The Forgotten Son, Night of the Intelligence, What's Past is Prologue, The Note)
He fathered Matthew in 1900, and in 1902 his wife fell pregnant with a second child who they called Gordon. It was some thirteen years later that Alistair began to realise that Gordon was not, in fact, his son, but the result of a night of indiscretion between his wife and brother. He never, however, spoke of this openly with either party. (PROSE: The Note)
During World War I, he worked British Intelligence in Russia but was forced to return to the United Kingdom in the aftermath of the Russian Revolution of 1917. One of his colleagues was Bertie Stopford, the then British ambassador to Russia. (PROSE: The Wages of Sin)
On 26 December, 1917, Alistair visited the Carmunnock police station, after Gordon had tried to enlist in the army underage. While picking up his son, he met Archibald Hamish Lethbridge-Stewart, his brother, and the two of them had a drink to celebrate their offspring's futures. He was concerned that Gordon was looking more like Archibald as he grew, but was glad Gordon had not realised why. (PROSE: What's Past is Prologue)
He became a grandfather in 1925 to James Lethbridge-Stewart and again to his namesake, Alistair Gordon Lethbridge-Stewart, in 1929. (PROSE: The Forgotten Son) Another account said that he became a grandfather when Alistair was born in India in 1930. (PROSE: Blood Heat, Island of Death)
Lethbridge-Stewart frequently pressurised the younger Alistair into following his footsteps and joining the British military. For Christmas 1938, he gave Alistair a box of toy soldiers and then asked him, "What do you want to be when you grow up?" Alistair was not pleased with his present.
After his eldest son was confirmed as Missing in Action in May 1945, Alistair received a letter from the family solicitor, who had held the letter since April 1938. The letter told him that Gordon had long since learned that truth of his biological father, but regardless he still considered Alistair his real father. After this, Alistair decided that perhaps it was time to talk to Archibald and reveal the secret to their offspring, and their grandchildren. (PROSE: The Note)
Behind the scenes
As confirmed by Mark Gatiss, the character of Archibald Hamish Lethbridge-Stewart, whom he portrayed in TV: Twice Upon a Time was intended to be the grandfather of Alistair Gordon Lethbridge-Stewart. However, the episode itself does not explicitly state their relations.
However, Andy Frankham-Allen, as Creative Director of the Haisman Estate, made it clear that neither the Haisman Estate nor Henry Lincoln, copyright owners of the Lethbridge-Stewart IP, consider the character as the grandfather, but that their official stance is the character is the grandfather's brother. Since that time, however, Frankham-Allen convinced the Haisman Estate to reach a compromise, and as a result it was revealed in the short story, The Note, that Gordon Lethbridge-Stewart was the illegitimate son of Archibald. Whether this means the Brigadier is aware has yet to be revealed.