Happy-Go-Lucky is being widely touted as a departure for UK filmmaker Mike Leigh (Secrets & Lies, Naked), but there's nothing new in its sentimentalising of the British working class. It's a curious film - part froth-and-bubble and part first-rate drama - but the more substantial scenes make it well worth seeing.

The protagonist is Poppy (Sally Hawkins), a 30-year-old London primary school teacher who is so relentlessly positive, hyper and chirpy as to be infuriating - though we're clearly supposed to love her. Poppy is the kind of person whose instant response to the theft of her bicycle is to chuckle, "I didn't even get a chance to say goodbye". She works wonders with the kids, whoops it up with her female friends, radiates ideological soundness and jokes manically with strangers no matter how uninterested they might be. So far, so unpromising - until Poppy starts taking driving lessons. Her instructor Scott (Eddie Marsan) is her polar opposite: an uptight, intolerant and scarily paranoid man who is constitutionally incapable of smiling. The more that we see of Scott, the weirder he seems, not least on account of his ideas, which are an amalgam of crypto-fascist conspiracy theory and the occult. He's not nice, but he's interesting, and Marsan is stunningly good in the role. He steals the show, though there are a couple of other creditable performances, notably by Alexis Zegerman as Zoe, Poppy's sardonic but warm hearted friend and housemate.

Much of Happy-Go-Lucky is groaningly predictable, and the "love interest" subplot which sees Poppy charmed by a handsome social worker is particularly so. Some of the dialogue is too self-consciously worthy, the nadir being reached in a tendentious pub conversation about social problems. But when it's "real", Happy-Go-Lucky is utterly gripping.