There is a wonderful moment in Longford when the ageing Earl, played by Jim Broadbent, and his wife Elizabeth (Lindsay Duncan, left, with the real Longfords inset) are sitting in bed surrounded by porn magazines. Frank is railing against the “Pavlovian” effects of such stuff on young boys. “I’m afraid I’m with Marylin Monroe on this,” says Elizabeth, gently. “When asked what she thought about sex, she thought for a moment and said she felt it was here to stay.”

Elizabeth, a woman whom Duncan says “could read the world more efficiently than Frank”, often speaks for the viewer, voicing our reactions to her husband’s obsessive, Quixotic campaigning — on parole for Myra Hindley, against pornography. The scene also illustrates the way that, in Duncan’s words, Morgan “uses the marriage as a device”, making him more human, more normal.

When I meet Duncan after the press screening, she is full of admiration for Elizabeth. “She wasn’t a public figure, but she was such a heavyweight. She was a highly successful biographer. She could have had a political career but she didn’t because of the marriage and the family. They were both incredibly intelligent, active, engaged people. But she found him incredibly difficult. Even that hugely successful marriage was complex. They both had to face aspects of him that she found impossible, frustrating.”

Jon Snow, who knew the Longfords well, was emphatic about the accuracy of her portrayal. Their grandson, Tom Pakenham, though a small boy during their old age, also says that he saw clear aspects of his grandmother in the performance, particularly her role as “a massive pillar of strength” within the family.

If the Longfords’ marriage was a complex and eventful journey, you could say the same of Duncan’s career. I first met her 37 years ago in the drama society at King Edward VI’s Boys School, Birmingham, later to become the model for King William’s in Jonathan Coe’s The Rotters’ Club. Two committed English masters organised seemingly endless drama productions and began importing girls from our sister school. Beautiful, blonde and with a wicked sense of humour, Lindsay might have been Cicely Boyd (though the two boys with whom she became life-long friends were gay). One, the future playwright Kevin Elyot, was a close friend of mine, so I saw Duncan quite often for a while. Kevin and I recall her doing superb cameos in a Dylan Thomas production.

“I had a wonderful English teacher called Kate Flint,” she says, “but we always did Shakespeare. The boys’ school were doing really challenging contemporary work. It feels that I was always hanging out there, and people like Kevin knew about things called drama schools. All the boys seemed interesting and very cool.”

Mainly I remember her constant laughter and joking with Elyot: silly voices, gentle teasing and camp humour. A few years ago my mother dug out a postcard she once sent me which began “Dear Cuddle Paul”. If she had wanted, she could have been a superb comic actress. Duncan’s first TV role was as a maid called Scrubba in Up Pompeii. She has just finished filming the second series of the BBC/HBO co-production Rome, and you might think that her role as Brutus’s scheming mother Servilia has brought her full circle. Only Lindsay could deliver a line such as “a large penis is always welcome” while accepting the gift of a naked slave and make it sound like a duchess complimenting the housekeeper on the floral arrangement.

She may not have achieved the international status of a Judi Dench or a Helen Mirren, but her record on stage and television is exceptional, thanks, in part, to her association with Alan Bleasdale (GBH; Jake’s Progress), Stephen Poliakoff (Shooting the Past; Perfect Strangers) and Harold Pinter (Celebration; The Room; Ashes to Ashes; Old Times).

Researching Elizabeth Longford, she was able to take advice from her friend Antonia Fraser on how to portray her mother. It is a superbly measured performance and a far cry from those giddy days in Birmingham. As she says as she leaves for the limo: “Who’d have thought it?”

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