Simon Block's uneven but hugely entertaining new play Chimps (Hampstead Theatre) opens on a Saturday afternoon as a young couple walk into the kitchen of their shabby, newly purchased north London flat. As they unpack the week's shopping, their niggling conversation reveals that past happiness as art-college sweethearts has become a distant memory.

Mark (Darren Tighe) resents the way his expectant partner talks about "my baby" not "our baby", and mocks her Scots accent. Why hasn't she showered him with praise for completing the first two letters in the children's alphabet book that will make his name as an illustrator? Graphic designer and sole bread winner Stevie (Ashley Jensen), is furious that he's blown pounds 38 of hers on champagne. So far, so conventional.

Then the doorbell rings, heralding the arrival of Gabriel, a salesman whose cryptic offer of a free estimate Mark had foolishly accepted earlier in the day, accompanied by his partner in patter, Lawrence. As soon as these two are across the threshold, Block's dialogue shifts into a slightly surreal gear and the evening takes flight.

Stevie retreats to prepare for a self-defence class, leaving Mark at the salesmen's mercy. His hopeless unassert- iveness, nicely brought out by Tighe's fidgety, teenager's body language, offers scant defence.

Fraser James's sharp-suited, gravel-voiced Gabriel is the straight man in this double act. Most of the time, he hovers in the background so as not to interrupt Nicholas Woodeson's Lawrence. Short, balding and stocky, Woodeson combines the smarm of a children's entertainer with the hope- over-experience determination of Del Boy.

Aphorisms, pet theories and non-sequiturs tumble from his mouth. "A word of advice about your children," he tells Mark, "don't get too involved." He is so confident of his facts that he'll "drink tea from a toilet" if Mark can prove him wrong. This is a fabulous comic creation, given the performance it deserves by Woodeson. But what, you want to know, is Lawrence actually selling? On the flimsiest of evidence - a few polaroids, a hastily- sketched diagram - it turns out that Mark and Stevie's front and back walls are "about to fall down". Unless, of course, they invest thousands in Excote, the miracle product that will safeguard them and their child for years.

We have to wait for Mark and Stevie to receive the offer they should refuse, but Block, for the most part, wards off impatience. His 1995 debut, Not a Game For Boys, had three male mini-cab drivers gathered in a run- down table-tennis club, and ping-pong is not a bad metaphor for the way Mark is batted back and forth between the manipulative Lawrence/Gabriel and the pleading Stevie, who sees through the men from the start. By the end the fault lines in the couple's relationship are rather wider than the supposed cracks in the walls.

Block could have given the pair's plight more emotional weight if Stevie were not off stage for so long (Jensen does well with a fairly thankless role) and if their arguments when left on their own felt less like soap opera. Director Gemma Bodinetz cannot smooth over the jarring shifts between these naturalistic exchanges and the stylised language of the salesmen's mind-games.

The Hampstead's literary manager has likened Chimps to David Mamet (the real-estate salesmen from Glengarry Glen Ross, presumably) and Kafka (the knock at the door leading to massive, improbable upheaval). Block's writing does not quite merit such lofty comparisons, but certainly makes Chimps worth seeing. Woodeson's performance makes it unmissable.

There's a rosier-tinted view of relations between the sexes in Divorce Me, Darling (Chichester Festival Theatre), Sandy Wilson's sequel to The Boy Friend. It takes place a decade later - we're now in the Thirties - but a trip to the Hotel du Paradis remains the panacea for all earthly woes, in this case 10-year itch, impending spinsterhood and financial ruin. Seen for the first time since its 1965 premiere, in Paul Kerryson's attractive, fizzing production, Divorce Me, Darling proves that life is still much nicer in Nice.

The original characters are swiftly reunited. Polly (Ruthie Henshall) thinks that her husband, the Hon Tony Brockhurst, has become fonder of their stately home than he has of her; sparks are rekindled between Polly and American Bobby van Husen; Bobby's wife gets jealous; his sister goes husband-hunting, and ... well, there's some short-lived confusion involving the President of Monomania and a fitness troupe.

Henshall displays the finest voice; Liliane Montevecchi - a Norma Desmond manquee if ever there was one - vamps it up to fine effect as the "mysterious" chanteuse Madame K; and Linzi Hateley contributes the most outrageous Dietrich pastiche since Madeline Khan in Blazing Saddles. Wilson's score proceeds merrily, cheeky, muted brass and dainty cymbal splashes to the fore. There are no showstoppers (Montevecchi's "Lights, Music" comes closest), plenty of tap-dancing, and no prizes for expecting a happy ending.

Heaven only knows what Polly and co would have made of the carnal and bloody acts perpetrated in Antonin Artaud's The Cenci. Artaud's 1935 version of the Shelley verse drama has been transformed into a remarkable 80-minute piece by the Italian composer Giorgio Battistelli and British director/ playwright Nick Ward as part of the Almeida Opera season.

Battistelli's music provides constant accompaniment, while his and Ward's text is through-spoken rather than sung. The actors' heavily-miked voices emerge as distorted whispers or echoes from speakers dotted around the auditorium. Ian McDiarmid's Cenci, a leering 16th-century Caligula, strides on to the giant, sloping wooden cross that serves as a diagonal stage and declares his intention to rape and murder his daughter Beatrice (Anastasia Hille, serene and saintly). She enlists the help of her stepmother Lucrezia (Kathryn Pogson) to kill him.

The family's destruction takes place against a background of sounds and film created by Studio Azzurro of Milan, with stunning ingenuity but minimal dramatic effect. When Hille describes an unnerving dream, the screens to the left and right of the cross are for some reason filled with a toad, pigs, a snake and a leaping hare. All too often, the projected images made you ask not only "How do they do that?" but "Why?"

At the Young Vic Studio, The Prince has an interesting premise. A weak politician, languishing in third place in the race to become governor of a crime-ridden, 21st-century metropolis, has his campaign galvanised by a mysterious spin doctor who appears to be a reincarnation of Niccol Machiavelli.

But then writer-director Simon Blake and the Changeinspeak company offer up an excruciatingly acted sequence of strategy meetings, press conferences and rallies: David Hare's The Absence of War re-made as a D-movie. So bad it's good? If only.

'Chimps': Hampstead Theatre, NW1 (0171 722 9301). 'Divorce Me, Darling': Chichester Festival Theatre (01243 781312), in rep to 27 Sept. 'The Cenci': Almeida Theatre, N1 (0171 359 4404), tonight only. 'The Prince': Young Vic Studio, SE1 (0171 928 6363), to 2 Aug.

Robert Butler returns next week.