Pipe Dream, Union Theatre

Rodgers and Steinbeck: sound unlikely? Well, self-proclaimed “family show” man Hammerstein may have baulked at words like "whorehouse" when he created a play for music out of Steinbeck’s Cannery Row and Sweet Thursday.

But by 1955 the R&H duo had already dealt with issues like miscegenation and ageism (South Pacific), domestic violence (Carousel) and slavery (The King and I), so Steinbeck’s north Californian coastal community of amiable social dropouts, drunks and whores might not have been totally unexpected territory. Tough Pipe Dream isn’t: "It’s a beautiful show," declared Steinbeck, "it lacks only guts". As it still does in the Union Theatre’s London stage premiere. But in director Sasha Regan’s hands the sterling virtue is an uncloying, low-key sweetness, which is something to be treasured.

Topical themes crop up in the first few minutes: marine biologist Doc, the neighbourhood’s fount of wisdom, tells his friend Hazel how studying the starfish may help mankind more than he thinks: everything is connected, all is one. The surface of the Californian ocean may fulfil your standard musical’s notions of beauty, but when the sea retreats, a tide pool reveals a brutal Darwinian struggle for existence (Regan’s cast acts out the human parallels behind the broken slatted screens of Elle-Rose Hughes’s simple but effective set).

The first two numbers, alas, aren’t exactly hits like their counterparts in Oklahoma!, another musical where nothing really happens. But there the tunes come thick and fast, as they did in an equally charming recent attempt to resuscitate a flop, Dear World. You’d be hard pressed to come out humming any from Pipe Dream, though its effect might be gauged from the fact that the post-show number I had on the brain was “Happy Talk” from South Pacific. The ecological theme quickly takes a back seat and everything depends on the will-they-won’t-they love story between diffident Doc and tough-on-the-surface Suzy, the vagrant who turns up with a hand cut from breaking a store window to steal some food. That it works a treat here is entirely due to the attractive and sympathetic performances of Kieran Brown (pictured right) and Charlotte Scott.

Scott goes for quiet and contained in an archetypally dreamy Rodgers and Hammerstein “I wish” song, “Everybody’s Got a Home but Me”. Brown, possessed of an effortlessly projected baritone and a wonderful way with dignified, intelligent dialogue, gets his own charmer: “The Man I Used To Be” incorporates a clever duet with a dancing shadow. Doc and Suzy’s tentative date-duet is beautifully understated by anticipating it as a Mexican track being played in the restaurant they’ve gone to, picked up by Doc as the kind of hypothetical lovesong R&H had already done so well in Oklahoma! and Carousel.

As in most of the duo’s best musicals, there’s an earth mother, brothel madam Fauna, originally written for Wagnerian soprano Helen Traubel. She never gets an anthem like “Something Wonderful” or “You’ll Never Walk Alone” but shimmies along nicely in “Sweet Thursday” and corrects the low self-esteem of our heroine - chaste occupant of her brothel before going off to live in the boiler pipe on the beach - with a lesson in cognitive behavioural therapy, “Suzy is a Good Thing”, starting as a very unconventional echo-duet. Virge Gilchrist has a tall, striking presence and a fabulous chest voice, though the missing last degree of finish to some of the phrases undermines her total authority. As always, Regan has picked the best of recently-graduated talent to sing and dance around her, among them David Haydn, Nick Martland - whose strong singing voice deserves better than his one number, "Thinkin' " - Rebecca Fennelly and Georgie Burdett, the latter oozing star quality and surely a name to watch (Fennelly, Burdett and Haydn flanking Gilchrist above). A couple of dodgy accents in minor roles are a small blot on the seaside landscape.

Lizzi Gee’s resourceful choreography makes a virtue of the Union’s small space and pianist/MD Christopher Peake together with an (uncredited) percussionist shows a subtle mastery of the sweeter idiom which is especially affecting in the underscoring - perhaps that should be underplaying - to the dialogue. The curtain may be abrupt, but sentiment rather than over-sentimentality wins the day, and this gem of a staging turns out to be another “family show” after all.

With The Color Purple running down the road at the Menier Chocolate Factory and Titanic at the Southwark Playhouse much admired by our own Edward Seckerson, London's South Bank is on a roll of musicals that New York's off-Broadway surely can't match at the moment. But we need to fight for the Union Theatre, a jewel of authenticity threatened by ruthless Southwark Council's drive towards creating office space they can't fill. There's a six month stay of execution, but then the campaign may need to begin all over again. This lovable and friendly place deserves our support.

• Pipe Dream at the Union Theatre until 31 August
• Read David Nice's blog, I'll Think of Something Later