Question: should you read the book before going to see the film of the book? In the case of an author as popular as Ian McEwan, the chances are that you’ve read it already. When his novella, On Chesil Beach came out in 2007, I read it straightaway. I couldn’t help re-reading it when I heard about the film, and that was my mistake, perhaps.

I went along to the Filmhouse with an open mind, curious to see how McEwan had adapted his own work for the big screen. From the very first scene, I was disappointed. The two young lovers crunch along the shingle beach of the title. They discuss music in a way that seems contrived, and has nothing to do with the story, using language that only students of Grade Five Theory would adopt.

In the book, the scenario is set up in an instant: the hotel room where newly-weds, Edward and Florence, are going to have to, despite huge apprehensions and anxieties, consummate their marriage. This is basically what it adds up to: a tense and dreadful anti-climax (well, premature climax) which is handled with agonisingly perfect timing by the novelist, if not the characters.

Because the film, once we’re in the Honeymoon Suite, is reliant on flashback there is a disjointed feel to the dramatic structure. As we dip in and out of the back-story, each time we return to the hotel room, it feels more like a tease than a build-up. Knowing it all ends in disaster, I find myself constantly wishing we’d go back to the past, since the courtship is portrayed rather sweetly, with none of the tension that we are being led to believe will attend the marriage bed.

Furthermore, there is something missing from the social comment that McEwan weaves through the novel. Sure, the young lovers first meet at a CND event in Oxford, and this gives a chance for us to hear Florence’s mother voicing her abhorrently (now obsolete) views on the Berlin Wall. But this is 1962, and the feeling that things are about to change big-style just doesn’t come across.

Then there is the weird depiction of Edward’s brain-damaged mother. She is portrayed (bravely by Anne-Marie Duff) as an eccentric but talented artist with a flair for inappropriate nudity. Since Florence, despite her immense sexual naïveté, is unfazed by this, and therefore adored by Edward’s family for the way she handles the crazy mother, Edward’s father (Adrian Scarborough) takes him to one side and advises him: “Marry that girl.”

Yet he omits the next line written in the book…

But this is the film, and the only obvious thing is that 1963, the year when, according to Philip Larkin, sexual intercourse began, is still a year away, and the Chatterley ban, even further off. While London is gearing up for the swinging sixties, our two ingénues are fumbling around on the satin sheets of a four-poster bed in a hotel by a pebble beach in Dorset.

Changing the tone of this review, there is little to fault in the performances of this poor couple. It is clear they have done their research (ie, read the book). Saoirse Ronan has got the well-mannered daughter of a bullying father (Samuel West) and Oxford Don (Emily Watson) down to a tee. Billy Howle, as Edward, copes admirably with attempting to express what is lacking in the script.

In his writing, McEwan used omniscient narration to constantly switch between the viewpoints of each protagonist, but in a screenplay, the only way to emulate this is through post-synch (ie, thought) speech. Only when things are about to come to a head (sorry) is this technique used, as Florence recalls in her thoughts a line she read in a stiff-upper lip sex-manual, on ‘guiding him in.’

It seems almost comical, and the film teeters on farce. The last time I saw Saoirse Ronan was in Ladybird in a hilarious loss of virginity scene where she ended up with a nosebleed! But in On Chesil Beach it’s clumsily different. Despite the big build-up (which, in the book, is a real thing for poor Edward, having ‘saved himself’ for the great occasion) I’m pretty glad when Florence runs out of the hotel in shame and disgust at Edward’s inexpertise.

Where the film redeems itself as an adaptation of the book is in the scene on the beach which best mirrors the tense dialogue in the novella. This is make or break: will Florence and Edward find a way to get round their misunderstanding and embarrassment? In her hopeless innocence, Florence makes a suggestion that enrages her already frustrated groom, and the whole thing turns to shingle.

That should have been the end of the film. In the book, McEwan wraps up the story in a couple of pages, explaining how each went their separate ways, with a Thomas Hardy-like sigh of regret. But this is The Movies, and to my regret, the film resorts to a bitter-sweet schmaltz-fest. That missing line from the book when his father advised Edward to marry that girl is: “before she got away.” This then becomes the reason for the film’s soppy ending.

Having spoiled most of the plot, I’ll leave the rest to your opinion. Suffice it to say watching it will not spoil the book in any way: On Chesil Beach was written by one of our greatest novelists at the height of his powers. I’m not sure I can say the same of his screenwriting.

On Chesil Beach is screening at Filmhouse, Edinburgh, until 7th June.