Love across the class divide in the aftermath of the Great War unexpectedly adds to the list of civilian casualties in Eva Husson’s artfully composed period drama.

Adapted from Graham Swift’s slim novel by Alice Birch, co-writer of Normal People, Mothering Sunday revels in the sexiness and sensuality of bodies entwined in urgent defiance of that most unforgiving mistress: time.

Odessa Young and Josh O’Connor gamely shed their inhibitions as the upstairs-downstairs lovebirds, raising the on-screen temperature several degrees with beautifully choreographed couplings that require matter-of-fact full frontal nudity.

These rhapsodic scenes, concentrated into one fateful day in 1924, are visually ravishing and exert a strong emotional pull as conflicted characters try valiantly (and in vain) to conceal deep psychological wounds of the previous decade.

Swift’s lyrical prose – an orphan is told to count her blessings for being “comprehensively bereaved at birth” – translates elegantly through the compelling performances of the two leads and co-stars Colin Firth and Olivia Colman.

Unfortunately, Birch’s script dissipates dramatic momentum by flitting between the intoxicating amour fou and two other timelines to examine the far-reaching consequences of this secret assignation.

Glimpses of the future are insubstantial and Glenda Jackson is squandered in a fleeting appearance as the most mature and self-reflective incarnation of the heroine.

On March 30 1924 – Mother’s Day – tradition dictates that wealthy families give servants the day off.

Mr Niven (Firth) and his emotionally brittle wife (Colman), who lost their two sons in the First World War, take leave of their mild-mannered housemaid Jane (Young) and cook Milly (Patsy Ferran) to spend the afternoon lunching at Henley-on-Thames.

Milly catches a train home while Jane cycles to Upleigh, the sprawling estate of her secret paramour, Paul Sheringham (O’Connor).

In two weeks, the prodigal son will marry socialite Emma Hobday (Emma D’Arcy), whose heart was promised to Paul’s older brother – another casualty of the conflict.

Jane and Paul spend precious time in carnal ecstasy, discussing the crushing weight of expectation on his shoulders as the surviving Sheringham boy (“I’ve got to get married, become a lawyer…”) and her fanciful dreams of becoming a writer.

Their tryst is blissful yet brief because Paul is due at the riverside lunch with the Nivens, his parents (Craig Crosbie, Emily Woof), Emma and her parents (Simon Shepherd, Caroline Harker).

When he leaves, Jane’s life of regimented service spins off its axis.

Mothering Sunday seduces our attention whenever Young and O’Connor trade whispered words and yearnful glances, then stumbles as time shifts and grief subsides.

Cinematographer Jamie Ramsay conjures striking tableaux of physical intimacy, inviting us to linger with Jane and Paul in their bubble before it bursts and reality crashes in.

Jane is mired in the past. For Husson’s picture, that’s a blessing.