Growing up, Celia Imrie’s mother Diana would tell her daughter tales of aristocratic forebears. “She did try to tell me how we were related to William the Conqueror,” says the actor at the start of her episode of Who Do You Think You Are?, “but I just found it too uncomfortable, I didn’t want people to think I was posh.”

So why does Celia, star of Calendar Girls and Victoria Wood’s spoof soap Acorn Antiques, want to find out about her family history? She’d be “shocked but intrigued” to find a criminal, plus she wants to see if any ancestors share her 18-year-old son Angus’s passion for politics.

Celia’s first visit is to a cousin, Patricia, where she sees a family tree filled with aristocratic names. The first of these whom Celia researches is her 8x great grandfather, William, Lord Russell(1639-83). A Protestant Whig politician during the autocratic Charles II’s reign in the 17th century, a time when religion and politics were inextricably linked, William was a key figure in trying to push through an ‘Exclusion Bill’. This sought to prevent Roman Catholic James, Duke of York, from succeeding his brother, who had no legitimate heirs. Protestants feared James’s accession might lead to Britain becoming a Catholic country.

This was a dangerous move, as became clear when William was accused of plotting to kill Charles and James. Was this true? It’s difficult to know considering William was sentenced to death in a propaganda trial. He was beheaded, but is now commemorated as “a lover of constitutional liberty” by a plaque at Lincoln’s Inn Fields. He was also posthumously pardoned when William of Orange, who brought constitutional monarchy to Britain, took the throne in the Glorious Revolution.

From where did William draw his personal courage? One answer may be in the example of his grandmother, Frances Howard. Married as a teenager to the Earl of Essex, she hated the match as letters from the era show. Remarkably, she decided to have the marriage annulled, claiming her husband was impotent and that she was a virgin, and submitting herself to examination by midwives and matrons to prove so. Essex, it’s said, countered the impotency claim by showing his friends “the sufficiency of his matters”.

This didn’t work. Frances remarried, to Robert Carr, the Earl of Somerset, a favourite in the court of James I. But more scandal lay ahead, when Frances was accused of plotting to murder Sir Thomas Overbury, who had tried to persuade Somerset not to marry her. She pleaded guilty, but insisted her husband played no part in the crime. It made no difference, he was sentenced to death.

Both were later pardoned, but there’s evidence that Somerset blamed his wife for ruining his career and the two were forced to live for several years in the Tower of London. The couple’s daughter, Anne, would marry the Duke of Bedford.

Celia has found both a principled politician and criminals in her family tree.“I’m so proud I’m not related to wishy-washy people drinking tea all over the place,” she says. “Quite the reverse.”