Been So Long, Young Vic, LondonThe Mountaintop, Theatre 503, Battersea, LondonThe Beautiful Journey, South Yard, Naval Dockyard, Plymouth

A Royal Court drama has been turned into a swaggering, hip musical in which lovelorn lowlifers break into torch songs when you least expect it

It's 11 years since the Royal Court premiere of Been So Long, Che Walker's rollicking play set in a Camden bar where comic losers, studs and mouthy ladettes cross paths – variously cruising for a bruising, a leg-over or more romantic liaisons.

The current Been So Long has been a long time coming, but now, in an inspired move, that straight drama has been turned into an updated, hip and humorous musical: a boisterous hit for the Young Vic (co-producing with English Touring Theatre).

Walker's self-directed production – with its predominantly black cast, including the soul star Omar – endows the genre with street cred. Songwriter Arthur Darvill's numbers range through blues, funk, reggae and hip-hop, all performed with soaring brio, aplomb and wit. The barman (Omar) and his lovelorn customers reach tongue in cheek for large handheld microphones – stowed in niches in the set – whenever they feel a torch song coming on.

Visually, too, this staging has a sleek intimacy. The bar (designed by Dick Bird) is a darkly gleaming basement dive, with glinting bottles and a staircase that spills, curvaceously, down into the audience. The front rows have been replaced by tables and chairs, cabaret-style.

Up above, in the musicians' gallery, Darvill and his band (bass guitar, drums and keys) perch with a trio of voluptuous backing vocalists. Down below, Naana Agyei-Ampadu struts her stuff as the outrageously brassy Yvonne, farcically tussling with Harry Hepple's scaggy Gil. Meanwhile, Cat Simmons's wary Simone falls for Arinze Kene's super-hunky, surprisingly tender Raymond.

Walker joyfully captures the ballsy swagger of these lowlifers, with the women more than a match for the men. Their lingo is flamboyantly quirky as well, fusing contemporary slang with flourishes of arcane eloquence.

Been Too Long could be faster paced at its start and finish, where it becomes slushy. There are flat patches. Nonetheless, this is a neat chamber piece compared with Walker's sprawling Camden epic, The Frontline. And it adds to the wave of productions currently enlivening the London stage with terrific, leading black performers.

That wave continues with Theatre 503's outstanding fringe premiere of The Mountaintop, starring David Harewood. In this historical-fantastical two-hander by American newcomer Katori Hall, it's the night before Martin Luther King's assassination in 1968. Harewood's MLK is holed up in a dingy Memphis motel when a young black chambermaid knocks at his door. She brings the coffee which he ordered, and has a bunch of surprises up her sleeve.

Hall combines a teasing seduction with a fierce political debate about how much King's "bourgeois" non-violent protests are going to achieve. The plot twists are bold too, as the maid, Lorraine Burroughs's Camae, starts to look suspiciously like an FBI agent, then, surreally, like an angel.

This playwright certainly doesn't fight shy of criticising the sainted civil rights leader, depicting him as a passionate reformer but also wearily embittered, and a casual womaniser on the side.

Yet he's a dramatically richer character for that. Pacing around his pokey suite, Harewood manages to seem both larger than life – bursting with charisma – and humorously real. The closely observed detailing is a joy, right down to his sniffing his rank socks.

When alone and avidly running through his big speech about America's aggressive arrogance, Harewood splendidly captures the swelling vibrato of the old-style preacher man. By contrast, he has a lovely laughing warmth when he relaxes with Camae. Burroughs is superb, in turn: mercurially innocent and sassy.

Some of the play's transitions are strained, especially when Camae dresses up in King's jacket and shoes to launch into her own rhetorical tirade. The allusions to President Obama –the dream-fulfilling orator to come – are also obtrusive. All in all, though, this is an exhilarating debut, and director James Dacre is another name to watch. What about a West End transfer to, say, one of the Young Vic's studio spaces?

Lastly, the environment has been shot to hell by climate change in The Beautiful Journey, a futuristic folktale devised by Wildworks, with support from Plymouth's adventurous Theatre Royal. I caught up with Bill Mitchell's roving troupe down at the city's old naval dockyard.

Wildworks creates wonderful raggle-taggle amphitheatres out of recycled junk and, for this al fresco promenade piece, they've created a shanty town-cum-cabaret club on the wharf, with multi-storey scaffolding and chintz curtains, plastic-bottle chandeliers and windmills made from tea trays.

The trouble is the script is far less brilliant, cobbled together from garbled Greek legends. The cabaret-queen Kalypso and the weather-forecasting witch Kassandra are rival sisters who eventually let a beached sailor and girl-heroine row away to seek a land where some bees have survived.

The acting is ropey and whole chunks are inaudible. Still, the gypsy band provides vibrant compensation, and Kassandra's aerial swoop is spectacular, swinging from a crane that flies her over a gigantic shipbuilding shed to gaze out to sea.

'Been So Long', Young Vic (020-7922 2922) to 15 Jul, then the Latitude Festival, Southwold (01603 660352) 16 to 19 Jul; 'The Mountaintop' (020-7978 7040) to 4 Jul; 'The Beautiful Journey' transfers to Oceana, Wallsend (0191-454 1234) 28 Jul-8 Aug