I wanted you, you didn’t want me.”

It’s a powerful moment that goes to the heart of the difficulties any filmmaker has when approaching Lennon. When you have someone who could be so heartrendingly eloquent about their own life, what are you bringing to the table?

In the case of Nowhere Boy, not too much that is outstanding but a lot that is seriously impressive. One to file under the latter category is the performance of its young star, Aaron Johnson, who plays the young Lennon as he begins to kick against his surroundings.

When Taylor-Wood’s film begins, Lennon is in trouble at school - “You’re going nowhere,” says the head, getting the title check in early - and his aunt Mimi (Kristin Scott Thomas) is none too impressed with his surly behaviour either.

It’s at a family funeral that he sees his mother, Julia (played by Anne-Marie Duff), who gave him into Mimi’s care when he was five. It’s the first time he has seen her since he was a child, and naturally believes that some great distance, or other hurdle, is responsible for her lack of contact all these years. When it turns out that she has been living nearby all along, Lennon is alternately puzzled, furious and entranced by the glamorous Julia, an attitude that would stay with him forever.

Such is Julia’s charm, and Lennon’s willingness to be his mother’s son, that the two hit it off immediately. She is fun, flighty, she plays the banjo and tells him he is her “dream”.

She’s everything, in short, that the staid Mimi, with her classical music and antimacassars is not.

Duff puts in a beautifully measured performance, resisting the temptation to make Julia either an angel or a demon.

We can tell from the smallest gestures, the way she looks at her son wide eyed in wonder, the way she shrinks at the mention of the formidable Mimi’s name, that this is a woman with a difficult story to tell. Scott Thomas’s Mimi, in contrast, is a character lacking in shades of grey.

Nowhere Boy, for all the directorial self-assurance on display, can’t quite make up its mind about the relationship between Lennon and Julia. There is one moment, when the film cuts between the two lying on a sofa and Lennon with a girlfriend, that strays in an awkward direction. Awkward because it is not clear what Taylor-Wood is trying to say. That Lennon was naturally comfortable around females? That beneath the wild rocker he was always a gentle soul?

The film is on surer ground with its lovingly realised portrait of Liverpool suburbia in the Fifties.

Seamus McGarvey, the cinematographer on Taylor-Wood’s acclaimed short, Love You More, but better known for his work on Atonement, The Hours and Charlotte’s Web, makes Liverpool look like a shiny new penny, just waiting to spend its riches in the coming Sixties.

The interiors are inch perfect, from Mimi’s front room to the music shop where she and Lennon buy his first guitar.

As Julia and John grow closer, the youngster begins to demand answers to the many questions he has been storing up, the chief one being: why did you leave me?

The screenplay by Matt Greenhalgh, who wrote Control, Anton Corbijn’s masterly portrait of Joy Division’s Ian Curtis, takes its time to answer that question. Such a long time, indeed, that by the time the truth is revealed, what had seemed so intriguing and important has lost much of its impact, despite emotions running as high as the birds on the Liver building.

Nowhere Boy is more conventionally biopic than Control. There’s a determination to tick boxes and tidy up as we go along, in contrast to Control’s steadfast focus on Curtis and letting all the other pieces fall where they may. So, in Nowhere Boy, we have “the big meet” between Paul and John, Lennon asking if his soon to be bandmate wants a beer only to get the reply, “I’d love a tea.”

The Buckinghamshire-born Aaron Johnson, 19, has the great disadvantage, for an actor portraying Lennon anyway, of looking a tad too healthy and rosy cheeked. Lennon was a Forties born child, he always looked scrawny, as a hungry young artist should, and in need of a hot meal.

Where Johnson gets Lennon spot on, however, is in attitude. The way he looks out from under his DA fringe and through those Buddy Holly specs, ready to take on the world, is pure rebel with a cause and a guitar.

As the picture leaves Lennon, we see the shape of the man he would become, all his passions, frustrations, and innate musical genius waiting to let rip.

Nowhere Boy (15)

Star rating: ***

Dir: Sam Taylor-Wood

With: Aaron Johnson, Anne-Marie Duff, Kristin Scott Thomas

Out on Boxing Day