The third and final episode of Mrs Wilson airs tonight, the completion of Ruth’s deeply personal miniseries about her grandmother Alison’s relationship with the husband she knew as Alec. They met while working together at MI6, when she was 21 and he was 48, living in London and raising two sons Gordon, Ruth’s uncle and Nigel, Ruth’s father.

After Alec died in 1963 of a heart attack, Alison discovered that he had three other wives, five other children, and that he had misled her about the nature of his work with MI6, his age, his income streams and even his name. (On his marriage certificate to his first wife he is listed as Alexander. Ruth and her family refer to him as Alec.) Alison kept these secrets until just before her death in 2006, when she bequeathed her sons the memoir she had written about their father.

The following year all of the Wilson children and grandchildren met for the first time, and have stayed in contact ever since. Within Ruth’s family, at least, the story has always been “vaguely funny, weirdly, because it’s so absurd.” So much so, that for the last decade Ruth’s immediate family had told her it would make a fantastic television show.

Mrs Wilson focusses on Alison’s story, with Ruth starring as her own grandmother. But Ruth consulted Alec’s four surviving children every step of the way, she says. “They read scripts,” Ruth says. “I made sure they were all happy with what we were doing and how we were dramatising various members of their family.”

The moment in the final episode of Mrs Wilson that brings Ruth Wilson to tears