Happy-Go-Lucky, 15

The veteran British director has taken a break from the grimmer side of life, and has created an ode to joy

Deeply entrenched cynicism, born of years as a critic, has taught me one thing. If a film is called Happy-Go-Lucky, either it'll turn out to be an obscure backstage musical, or it was named in the spirit of blackest, bleakest irony. You certainly expect this to be the case if the director is Mike Leigh, whose work – although it has raised its share of laughs – has more commonly gravitated towards the grimmer furrows of everyday experience.

But with Happy-Go-Lucky, no irony is intended. Leigh's new film is about a genuinely happy person. Attempting such a character is possibly the most thankless task in drama. But Leigh and his lead Sally Hawkins pull off the near-impossible in creating a figure who crackles with exuberance – an exuberance that, by the end, actually rubs off on you.

In Berlin, the role won this year's Best Actress award for Hawkins, who was also in Leigh's All or Nothing and Vera Drake. Her Poppy amazes us with her breezy spirit from the start. She cycles to work across London every day but isn't remotely frazzled when her bike gets nicked. She loves clubbing and falling about drunk with her sister and mates, goes trampolining in her spare time, and gets a huge kick out of her job as a primary school teacher: making paper-bag bird masks, then leading her class in boisterous flapping and squawking.

By now, you're probably expecting that Poppy will either have a nasty crash landing – look what happened to that nice Vera Drake – or reveal a dark side. But no. The only thing some viewers may find unacceptable about Poppy is that, at times, she could be accused of laying it on a bit thick. She can't resist trying to get a rise out of a glum bookshop assistant – "Having a bad day? Not till I showed up, eh?" – and flaps and giggles when palpated by a hunky physiotherapist: "Whatchamacallit! ding dong dilly dilly da da!" At such moments, Poppy's irrepressible hilarity tips very nearly into the Norman Wisdom bracket. I doubt there's a single viewer who won't at some point want to strangle the woman. She's infuriating, but in the way that Eric Rohmer's gabby, flustered heroines can be – and, unlike them, without a shred of neurosis.

One person who disapproves of her is driving instructor Scott (Eddie Marsan), whose molecular-level acidity seems to have formed a nasty precipitate on his teeth. Horrified by Poppy's inability to concentrate and by her impractically fancy boots, he seethes and snaps: "You celebrate chaos!" At first, you almost sympathise with him – then watch in horror as his warped, soured personality emerges in full. Fixated on ludicrous methods and mnemonics ("En-Ra-Ha!"), he reveals bilious depths of paranoia, racism and resentment. Marsan's Scott is a magnificent creation – magnificently funny, but also terrifying and extremely acute in its pinpointing of a certain fear-stricken mindset.

Poppy may inhabit a world that's nowhere near as happy as she is, yet her positivity and practicality help to make life liveable for others. She's clearly a compassionate and more than competent teacher. Realising one of her pupils is unhappy, she takes action that results in her meeting a simpatico social worker (Samuel Roukin), leading to possibly the single most convincing, no-nonsense, outright charming flirtation scene I've seen in cinema.

The film touches on all the themes – hope, hedonism, female camaraderie – that you'd expect in a generic "women's picture", and Happy-Go-Lucky stands every chance of pulling in the Bridget Jones audience and showing how it's done. But Leigh also ventures into unexpected areas: the register shifts dramatically in a very dark scene, as Poppy meets a disturbed homeless man. He's played by Stanley Townsend in a brilliantly painful performance, tics and broken language hinting at a hard-luck history we can only guess at. We're inevitably blindsided by this seemingly incongruous episode – what is Poppy after in accosting him, and does even she know? – but the shift brings to the film a subtly more complex perspective.

For all its lightness of touch, Happy-Go-Lucky is as serious and resonant as any of Leigh's films. Among other things, it's an essay on education, on the way that some people use knowledge to free themselves and others, while some chain up their psyches with useless systems and false expectations. There's a debate going on here about method and chaos that implicitly reflects the dynamics of Leigh's own dramas and his singular methods of developing them.

Leigh operates here on a much smaller canvas than in recent multi-stranded pieces such as Topsy-Turvy and Vera Drake. Making the most of its intimate scale, the film sublimely reworks that TV sitcom staple, the disastrous driving lesson, with two people squeezed into a glass-and-metal capsule, one chipping away at the other's sanity. The upbeat feel is enhanced by Dick Pope's photography, with its vibrant palette. Relishable performances include Karina Fernandez's priceless cameo as a flamenco teacher; Kate O'Flynn and Caroline Martin as Poppy's contrasting sisters; and, sustaining a beautifully developed thread of level-headedness, Alexis Zegerman as Poppy's flatmate Zoe, surely the most substantial best-buddy role ever seen in cinema. It's Zoe who sums up the film's gist when she says, "It's hard work being a grown-up, isn't it?" It certainly is – and it's often hard work watching films about grown-ups – but Happy-Go-Lucky goes a long way towards lightening the load.