© Johan Persson

They do indeed do things differently in the past: there, in 1953, LP Hartley's The Go-Between was a novel. The story of a man looking back at a traumatic youthful summer in 1900 when he became a naive messenger in an illicit love-affair between an upper-class young lady and a neighbouring farmer, it became a film (scripted by Harold Pinter no less) in 1971. And, in as recent a past as last year, it was made into a BBC drama.

Now, The Go-Between is a musical – and, in a smart casting coup, musical legend Michael Crawford plays the older Leo Colston, finally facing his demons. He wanders around the scenes of his youth as he remembers it all, which Crawford makes appear less hammy than it sounds. His voice is still sweepingly emotive.

But it's not exactly clear how the story benefits from Richard Taylor and David Wood's musical version. Pretty, expressive accompaniment is proved by just a piano, played onstage, and while action and dialogue are sung, there are no big numbers, or even many delineated songs per se. It's delicate, and unusual, refreshing even; full credit to musical director Nigel Lilley, who plays every note. But it's also more like a film score that's got ideas above its station: I wondered if telling this story of understatement, secrets and subtext would be easier if the cast could talk to each other (or not), instead of warbling near-constantly about breakfast and cricket.

But then, even as a play The Go-Between might struggle. Its plot twists on class and on place: the grand country house vs the farmyard, a cricket match between house and village, abandoned outhouses and erotically charged swimming holes. Designer Michael Pavelka's set is suggestive – a wonky drawing room, with grass sprouting between the chairs – but never exactly evokes either the swoonyness of an English summer, nor the subtle cadences of class division.

Tonally, the trick is to strike the balance between sun-drenched nostalgia and ominous foreboding. Here, we're too much bathed in the former. The show is significantly better in the tenser, darker second half when the stakes finally rise (although Issy Van Randwyck as an uptight mother swerves dangerously close to pantomime villain). But the damage has been done by an overly long first half which is far too dreamily elegiac for a snobbish Edwardian milieu, where young boys say things like "simply ripping" and "hard cheese" and everyone wears frothy white lace. There are a lot of schmaltzy songs about perfect summers and becoming a butterfly. I'm sure many audiences will find it sweet; I found it cloying.

William Thompson played the young Leo on press night: a 13-year-old boy, rudderless since the death of his father, who goes to stay with his posher pal and falls for his radiant older sister, Marian. Thompson's voice has a moving purity, and he shoulders a big part with impressive ease. Stuart Ward makes for a suitably hunky farmer Ted, although there's not much sense of either danger or vulnerability there. Gemma Sutton as Marian is all sweetness and light, with a delightful voice. But this character isn't just a innocent girl, and Sutton could use a spikier edge (remember beautiful cruelty Julie Christie brought to the part?).

For too long in this musical, there seems to be little drama – just a harmless schoolboy crush leading to the exchange of a few love tokens. Everything seems breezy, which makes it hard to credit a 63-year-old man being so haunted by events. Even when characters do hoot "remember!" at an agonised Crawford like ghosts of Christmas past, it's smirk-inducingly melodramatic. While things begin to grip and twist in a tauter second half, it's too late to make this feel like anything more than a mild-mannered costume drama with singing.

The Go-Between runs at the Apollo Theatre until 15 October.