A Little Night Music, Menier Chocolate Factory, LondonThe Family Reunion, Donmar Warehouse, LondonIn a Dark Dark House, Almeida, London

Sondheim's sex comedy with songs falls flat, and T S Eliot's verse play is bizarre. But Neil LaBute's study of child abuse hits home

I suppose A Little Night Music as opposed to A Whole Load has to be a good idea. Trevor Nunn's previous musical extravaganza – last April in the West End – was Gone with the Wind: an epic bore. So, to find him now staging Sondheim's belle époque sex comedy, as a chamber piece, on the London Fringe, sounds refreshingly frugal.

We're not talking shoestring budget here. Living up to Sondheim's droll lyrics and waltzing operetta-style score, the period costumes and lace parasols are elegant. However, the set has low-tech simplicity: just age-mottled mirrors covering the walls.

A leafy backdrop is added when the old flames Desirée and Frederik (Hannah Waddingham and Alexander Hanson) meet on a Swedish country estate. They crave a tryst, but his petulant teenage wife, Anne, is in tow. So is his rival, Count Malcolm.

The Joanna Lumley of musical theatre, Waddingham is a statuesque blonde investing Desirée with wit and warmth. In her duet with Frederik, "You Must Meet My Wife", she slips in her wry asides between the gritted teeth of a polite smile. Hanson portrays Frederik as fundamentally decent, if amorously confused. Meanwhile, Alistair Robins as the bullish Count thrusts bouquets at Desirée as if he's drawing pistols.

In spite of its fun moments, the evening goes flat. Jessie Buckley's Anne is a tiresome little featherbrain, and Waddingham's "Send in the Clowns" lacks poignancy because she looks so glam. The bigger problem is that the score grows prolix, especially with the dull number "Silly People" reinstated. A Little Less Night Music would have been better.

Having said that, sitting through T S Eliot's rarely aired verse play, The Family Reunion, you can't help wishing that Lady Monchesey and her gloomy houseguests would let their hair down and break into song.

No chance of that, of course, when Gemma Jones, as the aged matriarch, is keeping her upper lip as stiff as her sister's corrugated Marcel wave. Even the mansion's guilt-deranged heir – Samuel West's Harry, who claims he has killed his wife – is so larded with brilliantine that only his eyebrows shoot up in horror when he sees phantasms in the dining-room. Nevertheless, Eliot's conceit was to yoke together an English country house drama and a chorus more in line with Greek tragedy (stopping just short of song).

Alas, there is something faintly ridiculous about the starchy Moncheseys' choric turns, spasmodically launching into obtuse, synchronised poetry. Harry's spiritually in-touch Aunt Agatha (Penelope Wilton) also goes in for mystical chants: "May the weasel and the otter/ Be about their proper business/...Till the knot is unknotted/ The cross is uncrossed." "What a mad, mad play!" exclaimed the man next to me at the interval.

And yet Jeremy Herrin's revival is intriguing, performed in a dark panelled chamber, glittering with candelabra. The Eumenides who pursue Harry are envisaged here as ghostly small boys, like a memory of lost innocence. Wilton and West tackle Eliot's difficult speeches with a burning intensity. Hattie Morahan is vibrant too as Harry's childhood playmate, Mary: anxiously wide-eyed yet fierce with hopes of expiation or escape.

Whatever Eliot's shortcomings as a dramatist, he was a haunting poet. The bleak imagery running through this play – of lonely wastelands and dark corridors – seeps into your bones like an ague. It has the lingering menace of a nightmare. And maritally, of course, this writer knew about mental illness and despair.

At odds with its title, Neil LaBute's In a Dark Dark House is played out in a grassy glade in Michael Attenborough's superbly acted Almeida premiere: however, this verdant idyll is deceptive, as are LaBute's superficially affable characters.

We are, in fact, in the garden of a drying-out clinic. Moreover, the long-buried past – and, potentially, the future – of the two brothers who reunite here is bound up with pernicious child abuse. Steven Mackintosh's small wiry Drew is acting casual, like a superannuated teenager. But is he weaselly underneath? David Morrissey's quiet burly Terry seems rock-steady, but with alarming geysers of rage. Both try not to fall into temptation when a flirtatious 15-year-old, Kira Sternbach's Jennifer, enters the frame.

LaBute sets multiplying snakes of fear and doubt slithering through your mind. The sharp plot twists and the moral oscillations of his protagonists are unsettling – especially given the programme note where the playwright states that he was abused. Creepy, complex, highly intelligent, and not without some hopes of redemption.

'A Little Night Music' (020-7907 7060) to 8 March; 'The Family Reunion' (0870-060 6624) to 10 Jan; 'In A Dark, Dark House' (020-7359 4404) to 17 Jan