The new film version of Vera Brittain’s First World War memoir Testament of Youth comes to the nation’s screens this week with a 12A certificate. Beneath the opening announcement of this is an explanation of why some parental caution is necessary: “Contains scenes of bloody injury.” This is no exaggeration; director James Kent has spared little detail in presenting the grisly consequences of conflict suffered by those in the fighting and faced with unflinching equanimity by those such as Brittain, a Voluntary Aid Detachment nurse, struggling in almost impossible conditions to care for them, often in their dying moments.

In a key passage in the book, we learn how both groups coped, or tried to. Brittain writes: “Between 1914 and 1919 young men and women, disastrously pure in heart and unsuspicious of elderly self-interest and cynical exploitation, were continually re-dedicating themselves . . . to an end that they believed, and went on trying to believe, lofty and ideal. When patriotism ‘wore threadbare,’ when suspicion and doubt began to creep in, the more ardent and frequent was the periodic re-dedication, the more deliberate the self-induced conviction that our efforts were disinterested and our cause was just.

“Undoubtedly this state of mind was what anti-war propagandists call it — ‘hysterical exaltation’, ‘quasi-mystical idealistic hysteria’ — but it had concrete results in stupendous patience, in superhuman endurance, in the constant reaffimation of incredible courage.”

I read these words shortly before seeing the new film. Would it, I wondered, reflect the fierce anger that lay behind them? Would Brittain’s “passionate plea for peace,” which is how she described the book, survive on to the screen?

It does a bit. But most films set out primarily to entertain, and this unfortunately is one of them.

The story it tells — as many will know — has a strong Oxford dimension. The tale starts at Uppingham School where Vera’s younger brother Edward Brittain, to whom she is devoted, forms with fellow pupils Roland Leighton and Victor Richardson an inseparable trio known as ‘the Three Musketeers’.

Vera, who is struggling against paternal disapproval to gain a place at Somerville College, meets and falls for Leighton. He has a place at Merton but, instead of taking it, chooses to enlist and goes off to war. Edward and Victor both join up, too. All three of them are destined to die before peace comes.

At Somerville, Vera quits her studies after a year and begins a nursing career in her home town of Buxton, later moving to London, Malta — an important episode entirely missing from the film — and eventually to France (Etaples). She is back in London when the Armistice is declared and returns to complete her Oxford studies.

It was not until 1933 that Testament of Youth was published, containing — besides a powerful memoir of war — an account of the book’s genesis. A huge success initially, it declined in popularity as the Hitler war approached and all but disappeared from view until a Virago edition came out in the 1970s (when there was a BBC television production of it).

Mark Bostridge, Vera Brittain’s biographer, deals comprehensively with all this in his book Vera Brittain and the First World War (Bloomsbury, £16.99), which was written to complement the new film, on which he worked as a consultant.

How, I wonder, does he feel about the various errors of fact that scriptwriter Juliet Towhidi has introduced to heighten the emotional impact of the film?

These include a proposal of marriage that was never made by Vera (superbly played by Alicia Vikander) to the blinded Victor Richardson and Vera’s rescue from amid a pile of corpses of her brother Edward, as a prelude to her nursing him back to health (in which she did assist).

I wonder, too, why James Kent felt obliged to deliver such a clichéed nothing-but-rain picture of life in the trenches. “This afternoon is glowing with the languorous warmth of the dying summer,” Edward wrote in a letter home. “The sun is a shield of burnished gold in a sea of turquoise . . . It is a pity to kill people on a day like this.”