Shirley Williams sits at the head of a table in a large conference room in Lib Dem HQ. She will be 85 this year, but still has a finger in many a pie, most of which we’re not to talk about here, including the predicted wipe-out of a generation of her party’s MPs at this year’s election. It’s one of the reasons she never made it to see the Tower of London poppies. Too busy. She also had to dash to Russia where she is on the board of the Moscow School of Political Studies. ‘It is all about teaching people about democracy and has fallen under the frown of Mr Putin, which is why I had to go.’

What we’re here to talk about is her mother Vera Brittain, and the lives of the four young men Brittain chronicled in Testament of Youth, an unimprovable account of living through the Great War.

Roland Leighton, Edward Brittain, Victor Richardson and Geoffrey Thurlow: the names will chime for anyone who has devoured Brittain’s memoir. As an undergraduate who suspended her studies in Somerville to take up nursing, Brittain told of surviving while all those she cared about — including her fiancé Roland and her brother Edward — did not. Published in 1933, it drew heavily on letters, diaries and poems to portray with heartstopping immediacy what it was like for a generation to be doomed to death by their elders. It reported not from the trenches but from the homes and the bedsides of the inconsolably bereaved. The young men of her acquaintance, with the burden of history dumped on their shoulders, bloomed back into life at the stroke of her pen.

More than 50 years on, and well after Brittain’s death in 1970, Testament of Youth enjoyed a flurry of exhumations. Virago’s popular new edition of the book came in 1978, then in the following year there was a Bafta-winning BBC drama, and in 1980 a ballet by Kenneth MacMillan. And now, somewhat against the odds, there is a film, dedicated to the memory of the quartet and greenlit by Baroness Williams of Crosby.

As her mother’s literary executor, she — and Brittain’s biographer Mark Bostridge — had ultimate power of veto when Heyday Films (who also produced the Harry Potters and Paddington) asked for the rights. When she saw the first draft of the script she was all for hollering ‘expelliarmus’.

‘It didn’t make enough of my mother’s ambition to be a writer, and it didn’t reflect her commitment to become an opponent of wars in the future. The temptation to make a Hollywood romantic box-office success was very great. And that was the last thing on earth that I wanted because she would have been furious. It would have been a betrayal of her.’

So words were had and the romance with Roland was de-cheesed, and now Williams can’t holler her praises too loudly. But there were bumps along the way, the inevitable elisions and gauche rejiggings when a doorstopper covering more than a decade is pulped down to two hours. The book’s devotees may grind their gnashers, for example, at the near-disappearance of the saintly Geoffrey, and the placement of Brittain’s future husband George Catlin far earlier in her life than he actually appeared, and, most factitiously of all, Vera’s discovery of Edward’s still breathing body in a fresh heap of corpses at Étaples.

‘That’s dramatic licence, I’m afraid, and I raised that with them. I told them it was inaccurate.’ As for Catlin, her publicity-shy father, an academic who in the book bartered his identity down to a lone initial, ‘the question really is whether he should be brought into it at all. He didn’t wish to be seen as an appendage to a well-known author.’ This was a man who once told his daughter that ‘the hardest rival I had was a ghost’.

Williams also had to field Somerville’s protestations. Hers and her mother’s alma mater was cold-shouldered in favour of the sandstone quads of Trinity and Balliol. ‘It was made clear to me that they were very disappointed. The filmmakers said, fairly enough, I suppose, that the world thought of Oxford as gothic stuff.’

But the element Williams misses most is any allusion to Edward’s passionate musicianship. ‘They don’t make enough of that. I’ve actually got Edward’s violin in my study. It’s an artefact that was perhaps the most important part of him.’ Another treasured memento is a copy of the Gestapo blacklist: her parents — she a vocal pacifist, he a leading political scientist — were the only non-Jewish couple to feature on it.

Williams also struggled with a deeper reluctance to overlay the memory of Cheryl Campbell’s performance in the BBC drama. ‘She was so like my mother in many ways. You couldn’t get a cigarette paper between the two.’ Playing Brittain this time round is Swedish actress Alicia Vikander, whom Williams hasn’t seen as a pert young Danish queen in A Royal Affair. ‘I’ve come around to thinking that Vikander’s exceptionally good. But certainly it took me a little while to get used to the Scandinavian distance. She’s got a quality I came to appreciate, which is a face that expressed the most subtle emotions.’

Vikander certainly captures the earnestness. By Williams’s admission her mother was no bundle of laughs. ‘I don’t know if I’ve got a great sense of humour. My mother had virtually none. The white crosses were so planted in her mind it was almost impossible for her to see the world as funny. You just couldn’t be a distinguished writer without being pretty earnest. Look at the Brontës.’ As the film enjoys showing us, Brittain channelled all her frivolity into fashion. She was quite the clothes-horse. ‘Don’t look at me as an example,’ says her daughter, decked out in a frills-free tweed twinset.

What Williams really admires about the film is the friendships (‘brilliantly done’) and the sense of a young woman’s pioneering struggle for gender equality, plus the later glimpse of her internationalism. Brittain took the story of Testament of Youth deep into the Twenties to portray her growing conviction that the Great War must also be the last war. Williams is aware that her mother’s pacifism is not fashionable, and indeed some columnists have given Brittain’s stance towards the Third Reich a kicking as her story returns to national attention. ‘I didn’t agree with my mother about that. I concluded that Hitler was so evil that you couldn’t stop him even with the most dedicated pacifism — he would have shot Gandhi.’

The film, directed by James Kent, hits a lot of the right notes. It begins with Vikander walking numbly through the celebrating crowds on Armistice Day, her eyes at one point fixed directly on the lens as if requesting an audience with the future. You’d need very faulty faucets not to be moved by the moment Vera discovers Roland (Games of Thrones’s Kit Harington) has been killed.

The hope is that the film will send new readers to the book for a bracing dose of Brittain’s candour, unrivalled among female chroniclers of the war. ‘That’s the thing I most respect,’ says Williams. ‘She was an incredibly honest woman. She never softened the truth. Nor did she exaggerate it. Don’t forget we’re talking about the Edwardian age. People were not candid, especially women.’