Nanny McPhee (Thompson) arrives to help a harried young mother (Gyllenhaal) who is trying to run the family farm while her husband is away at war, though she uses her magic to teach the woman's children and their two spoiled cousins five new lessons.

An unfunny nanny who outstays her welcome.

Many critics turned up their toffee noses at Nanny McPhee but audiences embraced the fantasy featuring the most magical guardian angel since Mary Poppins and the film sold a healthy $US122 million worth of tickets in 2005.

Another film based on Christianna Brand's Nurse Matilda books was almost inevitable, but why the longish interval? The star/executive producer/writer Emma Thompson began crafting the story for the sequel while the first film was shooting and spent three years writing the script, according to the production notes.

After watching Nanny McPhee and the Big Bang, I’d have thought it would have taken several months, at most, to conjure up a thin plot involving obnoxious kids, a missing father, a pantomime villain and sundry animals including acrobatic pigs, goats, cows, a burping bird and a baby elephant. The result is a kids’ movie which I suspect will appeal to a very narrow demographic, from toddlers to 10-year-olds, offering very little wit or entertainment to adults.

There are familiar elements from the original as Ms Thompson’s grotesque nanny is again adorned with bulbous nose, false snaggletooth and warts, imperfections which start to disappear as her unruly charges learn life’s lessons while she bangs her magic walking stick.

But none of the characters is grounded in reality, laughs are rare and there’s a fleeting moment of pathos. Curiously, after the first half hour or so Thompson relegates her character to a background role, mostly seen flashing that silly tooth, nodding wisely as the ticks off the five lessons, or chastising her belching jackdaw Mr Edelweiss.

Set in wartime England – presumably WW2, although this isn’t specified – the story focuses on the harried Mrs Green (Maggie Gyllenhaal, sporting an immaculate English accent), who’s struggling to run the family farm and care for her three children Norman (Asa Butterfield), Megsie (Lil Woods) and Vincent (Oscar Steer), while her husband Rory is away at war.

Rory’s good-for-nothing brother Phil (Rhys Ifans, adopting an accent which seems to wander between his native Wales and the West Country) owns half of the farm and is trying desperately to persuade her to sell the property. It turns out Phil owes a lot of money to the local casino (there was a gaming facility in rural England in that era?) and if he doesn’t pay up, local thugs named Miss Topsey and Miss Turvey are threatening to remove his kidneys without an anaesthetic.

The blustering, blundering Phil is such a dim bulb that even the youngest viewer will figure out there’s zero chance of him succeeding, so there’s no tension in how that plotline plays out.

Nanny McPhee fortuitously turns up as Mrs Green is being driven mad by her posh niece and nephew, Celia (Rosie Taylor-Ritson) and Cyril Gray (Eros Vlahos), who’ve arrived from London and who take an instant dislike to her kids, and vice versa. Without revealing the film’s sole spoiler, suffice to say that Nanny, Cyril and Norman head to the War Office in London in a motorcycle with sidecar to seek the help of Cyril’s dad Lord Gray. Ralph Fiennes injects a welcome bit of gravitas as the cold, stiff-backed Lord Gray. Maggie Smith potters around to little effect as a dotty old woman who runs the local store. None of the child actors is especially good, and several sound as if they needed intensive work with a dialect coach. One sequence in which the pigs show their prowess at synchronised swimming is a rare highlight.

The film marked the feature debut of director Susanna White, whose background in TV dramas (Generation Kill, Bleak House, Jane Eyre) would seem to make her ill-equipped to handle this kind of candy-coloured confection.

White claims she was inspired by classic family movies such as The Railway Children and The Sound of Music which combined emotion and comedy, but this film comes up short on both counts.