John Carney’s Once became the sort of Cinderella story Hollywood would jump on in a heartbeat.

Costing less than the average London flat, the Irish underdog went on to win acclaim from audiences and critics alike (and an Oscar for Best Song), before transmogrifying into one of the hottest tickets in the West End musical scene. It was every producer’s dream.

Begin Again is, from the very poster, an attempt to make lightning strike twice. We know there’s a scruffy loser and a girl-next-door for him to fall in love with, and music is most definitely involved. In fact, Begin Again couldn’t be a more ironic title; this is basically Once with a bigger budget, albeit an indie feel.

Said budget brings in Keira Knightley as Greta, a British singer-songwriter who followed her heart to New York, and Mark Ruffalo as Dan, the scruffy A&R loser who wants to sign her and share her gift with the world. They argue over the authenticity of the pop star and whether Greta needs to doll herself up to be marketable, but such issues ultimately don’t matter and discussion segues into shots of New York for musical montages featuring cellos. James Corden, CeeLo and Adam Levine ensue.

If that all sounds insufferable, then you really ought to look away. But, to paraphrase 30 Rock, if you have two ears and a heart, then you could do a lot worse than this.

Every bit as unashamedly buoyant as Carney’s last musical, Begin Again sweeps the viewer up and won’t stop until it’s turned every frown upside-down. It doesn’t matter that the story hinges on this talented young woman’s Radio 2-ready songs bringing a jaded New Yorker back to life; people sing and play instruments on sunlit rooftops, so you have no choice but to get all Frank Capra. And ‘Lost Stars’ is a rather lovely ditty.

Ruffalo performs his usual cool-but-broken everyman, while Catherine Keener provides sturdy support as his flake of a wife. As Greta’s British BFF Steve, James Corden defies expectation by being more sweet than annoying, and even provides most of the movie’s laughs. As do, perhaps equally surprisingly, CeeLo and Levine – the former as a self-idolising rapper and the latter as a hilariously-bearded Mumford wannabe.

All eyes on Knightley, then, to see how she holds up in such company. For though she’s the biggest British export since PG Tips, she has struggled over the years to be accepted as a bonafide actor. Sadly, it’s safe to say that Begin Again won’t win her any new fans; Greta is the sort of fun-kooky-cute that feels self-conscious in the wrong hands, and in this regard Knightley is no Markéta Irglová.

Though she gives the film some of its more poignant moments (watching her song be performed by someone else), and seems genuinely to enjoy Corden’s company, the Anna Karenina star fares noticeably better when she’s playing literary heroines in Joe Wright adaptations. Hers is also the character most lumbered with corny script, such as the Mary Poppins-like subplot to improve Dan’s relationship with his daughter (Hailee Steinfeld).

However, all of this melts away when Greta starts to sing, revealing a whole other Keira Knightley: honey-voiced chanteuse. Where has she been and when will she be back?

The chief problem with Begin Again is that this Carney compares unfavourably to a former Carney. With its mix of bitter and sweet, Once plucked many a heart-string. From shaky cam to initial piano-guitar duet, it never felt less than authentic, and was all the more moving for its realness. Yet, for all its talk of risk and innovation, Begin Again is the polished-up studio version of a track that should’ve been a Black Cab session.

For all its flaws, Begin Again is the cinematic equivalent of a cup of tea and a hug. Ignore what it purports to be and accept it for what it is: funny, sweet and charming.

Released in UK cinemas on Friday 11 July 2014.