I Capture The Castle (PG),

The blooming English

Tim Fywell's adaptation of Dodie Smith's much-loved novel I Capture The Castle is a pert, good-looking entertainment that feels like a labour of love. The book, entangled in contractual toils for years after Disney hit the jackpot with Smith's better-known novel 101 Dalmatians, was finally freed from limbo after some clever horse-trading by her literary executors. Its particular strain of English shabby-genteel bohemianism offers great scope both to location scouts (dilapidated castles, glittering mansions) and to the wardrobe people (cloche hat meets top hat), while its golden autumnal colours and art deco designs lend it the brittle, elegant patina of Thirties railway posters.

Couched in diary form, the film strikes a tone somewhere between girlishly wistful and endearingly droll. The diary keeper is 17-year-old Cassandra Mortmain (Romola Garai), a bookish, thoughtful girl who lives with her family in a remote, run-down castle in Suffolk. Her father, James (Bill Nighy), is a writer whose early promise has failed him and left them on the verge of bankruptcy, while her stepmother Topaz (Tara Fitzgerald) likes to disport herself naked on moonlit nights. Cassandra's older sister Rose (Rose Byrne) dreams of a rich suitor who might rescue her from their impecunious straits, and her prayers seem to be answered when two American brothers, the Cottons, stray to the Mortmain's front door on their way to a manor house, which the older one, Simon (Henry Thomas) has inherited. Cassandra fears the worst after Rose overdoes the coquettishness very badly: "If only they could afford to send her to the cinema," she reflects of her parents, "then she'd have a better idea of how to behave."

In spite of this a neighbourly amity is sparked between the Mortmains and the Cottons. The benign matriarch Mrs Cotton (Sinead Cusack) adopts Mortmain as her new friend, while thanks to Cassandra's stratagems Simon pops the question to Rose, whose head has been turned by silk stockings, perfume and trips to Simpson in Piccadilly. (It could almost be a metaphor for our native film industry: British talent at the beck and call of American money.)

One of the funniest strands in the film is watching the youngest Mortmain, the precocious Thomas (Joe Sowerbutts) pore over the breathless letters Rose sends the family about her fabulous life in London ("I had to look up 'peignoir' in the dictionary," he sighs). The plot thickens when Simon spends an evening alone with Cassandra, whose life is thrown into turmoil on realising that she's in love with her sister's fiancé.

As Cassandra confides to her diary, "The thing we know least of all about is being women", a naivete conveyed with winning tenderness by Romola Garai. It's a very different performance from her spoilt show pony Gwendolen Harleth in the recent BBC adaptation of Daniel Deronda. Fywell's camera can make her look dowdy one moment and astonishingly beautiful the next, and in her portrayal of unrequited longing one senses that in this aspect at least Dodie Smith's novel has been precisely served. None of the other characters comes through quite so vividly, and in the case of the menfolk – Henry Thomas and Mark Blucas as the brothers, and even Bill Nighy as Mortmain – they barely come through at all. Until it hits the wall about two-thirds in and loses its jauntiness, I Capture The Castle is a perfectly agreeable bit of heritage cinema, Sunday evening stuff that won't embarrass anyone. It's only when Romola Garai is on screen that it looks likely to touch more interesting, tragicomic notes.

Mark Herman, a specialist in underdog comedy (Brassed Off, Little Voice, Purely Belter), now tries his hand at romantic comedy in Hope Springs, which from its title onwards inclines you to fear the worst. Colin Firth plays an uptight portrait artist who arrives in the New England town of Hope with a bad dose of jetlag and a bruised heart: his fiancée Vera has just decided to get married to someone else. I wonder if it has ever been any other jilted lover's experience to book into a roadside hotel where the manageress (Mary Steenburgen) immediately decides to matchmake you with a sexy blonde "careworker" (Heather Graham) who within 15 minutes of arriving has shed her clothes and done a little dance for Colin? The film has been adapted from a novel by Charles Webb (who wrote The Graduate), and I can only assume that this seduction scene worked better on the page than it does on screen.

With a sinking heart one realises that Herman has simply transplanted the shortcomings of Britcom – feeble writing, a reliance on farce, an air of desperate contrivance – into an American setting. Did he imagine that his script would undergo some wondrous alchemy in the process? I'm afraid the introduction of Minnie Driver as Colin's fiancée doesn't improve things. She arrives in a cloud of cigarette smoke to try and drag him back to Blighty (her marriage announcement was only a stratagem to "jolt him into action") but her breezy metropolitan hauteur and that curious peanut-shaped face just don't cut it with Colin any more.

I found myself feeling sorry for Driver, who by degrees has become one of the least popular actresses in the world. Taking on jobs like this won't help. As for Firth, his film career has never come close to matching his small-screen success, and that includes the pale reprise of his immortal Darcy for Bridget Jones's Diary. His natural mode is a certain lugubrious distraction, and his stiff-lipped wariness simply has no place in light comedy. Give the man his breeches and frock coat back – he's not just in the wrong country, he's in the wrong century.