The seven-day glitch: "My Week with Marilyn"

My Week with Marilyn intends to be 8½ or Day for Night as enacted by the 49% of the British film industry who weren't in Tinker Tailor Soldier Spy: it stocks a particular idea of quality in depth - even the bar staff here are played by someone or other from Downton Abbey - and stages an all-out assault on, if not the awards season, then certainly the 9pm Sunday night slot on ITV3. Its source is a memoir by Colin Clark (played in the film by Eddie Redmayne), a film-mad Home Counties lad who, in the summer of 1956, landed the position of third assistant director on the set of director-producer-star Larry Olivier's The Prince and the Showgirl; a gig that would bring him into the circle of none other than Marilyn Monroe at the height of her success.

There would perhaps be no greater contrast of screen personalities and acting styles than that between the Shakespearian knight (Kenneth Branagh here) and the blonde bombshell (Michelle Williams), and Simon Curtis's film has an intriguing culture clash at its centre: Marilyn brought sex and magic to the stuffy, parochial confines of Pinewood Studios, and a London that still wasn't quite swinging. She was also deeply insecure, of course, followed around by an entourage designed to protect her, and to keep the world from glimpsing what she saw as flaws. This entourage would include her husband, the writer Arthur Miller (er, Dougray Scott), the acting coach Paula Strasberg (Zoe Wanamaker), whose Methody prompts were entirely out of place on the set of fluff like Olivier's film, and eventually Clark himself, whom the memoir has us believe was personally recruited by Marilyn to serve as her confidant.

As a film about a film, My Week with Marilyn hits a peculiarly mid-Atlantic tenor: the Weinsteins signed the cheques, which accounts for the talent assembled here, but that talent brings with it a very British self-consciousness at the prospect of being around cinema myths and legends. (One comparison point would be with Richard Linklater's underrated, far less fussy Me and Orson Welles, which cast its Welles against the kid from High School Musical, and got on with the show.) It takes some while for the ear to adjust to the level of impersonation being attempted: Toby Jones and Dominic Cooper ape tough-talking US production chiefs, while Judi Dench mimicks Sybil Thorndike, and a desiccated Julia Ormond plays Olivier's then-wife Vivien Leigh; Scott's Miller has the unmistakable rasp of Sean Connery.

As Olivier, Branagh accepts the limitations of this type of production, and hams his way through the role of an ageing ham: there's a degree of skill involved - you have to know where the scenery is to chew it this thoroughly - though he's lucky that Adrian Hodges' script affords his exasperated Larry the lion's share of the funny lines ("Teaching Marilyn to act is like teaching Urdu to a badger"). As Marilyn, Williams is dazzlingly pretty, and - in her better moments - conveys a sharp, poignant sense of the the lost little girl who just wanted to be loved, but Curtis keeps forcing her into notionally cute Marilynisms (a wiggle of the hips, a finger to shushed lips) that again draws one of his performers into the realms of self-consciousness, and sometimes outright naffness.

It almost goes without saying that Williams cannot occupy the screen in the same way Monroe did, and in part that's deliberate: the film wants to show us a Marilyn removed of her glamour, and backing away from the spotlights into a dark psychological corner. Yet more critically, this being a British film tailored to a Masterpiece Theatre audience, sex - the very sex that Marilyn oozed - has to be stifled at every opportunity. Clark's fling with a costume girl (Emma Watson, hardly tested in her first two post-Potter scenes) is halted with a deeply square "wait a while, crocodile" when he goes for the second button on her blouse; his banter with Marilyn is sub-Hawtrey innuendo ("large packages", "tied up"), until she takes him skinnydipping and he emerges from the lake in a state of embarrassed tumescence. "Oh Colin, and you an Old Etonian and all," Marilyn purrs, in the script's falsest line: even the boners here have to be weighed down with the suggestion of class and good breeding, as though Clark were flying school colours from his bellend.

The film, indeed, turns on an instance of chivalry that is again self-aware, and somewhat difficult to buy whole: as this version has it, Clark was the only guy who slept with Marilyn while fully clothed. There are less conservative developments elsewhere, and the narrative allows the possibility of change for at least one of its characters: Olivier is last seen going off to work with John Osborne on The Entertainer, having taken Miller's advice on new, more radical forms of theatre. Marilyn, alas, simply has to be cast out, because no-one in the country - egads, not even Sir Olivier! - could handle her. Curtis's film is diverting, and often amusing - it'll fit that Sunday night schedule to a T - but it's also oddly throwaway, a reminder only that there's a particular type of British film that, unlike Monroe, will always be antithetical to a broader, more resonant idea of cinema. In its own haphazard manner, My Week with Marilyn goes to explain why nobody remembers The Prince and the Showgirl in the way they do Gentlemen Prefer Blondes, The Misfits and Some Like It Hot.

My Week with Marilyn opens in cinemas nationwide tomorrow.