Torben Betts describes himself as Alan Ayckbourn's darker, younger brother, something of a handicap, perhaps, when you consider he's had 14 plays produced, has written countless others and feels slightly hard done by for not having been picked up by the Royal Court or the National.

He could equally feel that not fitting in is a badge of honour for a good playwright, and Betts is undoubtedly that. Invincible, one of several Betts plays produced by the shamefully Arts Council-abandoned Orange Tree, where it premiered in March, is a blistering contemporary tragi-comedy masquerading as a traditional boulevard sitcom.

Ayckbourn and Mike Leigh are commonly cited as influences on this brutal social collision between middle-class Oliver and Emily (Darren Strange and Laura Howard), who have moved up north in flight from the recession, and their new working-class neighbours, Alan and Dawn (Daniel Copeland and Samantha Seager); but the wryness, darkness and bolshie humour has a sting of Willy Russell at his best, too.

Oliver, a laid-off civil servant, and Emily, are renting, and she won't marry him just to please his dying, bigoted mother. Children's toys litter the floor (as in Ayckbourn, we never see the children), William Byrd plays on the hi-fi, Emily's lighting joss sticks and the cat has vomited on the oregano.

Alan's cat, Vince, looms large as a bone of contention, and a subject of his secret paintings, which Emily openly excoriates in a cringe-making altercation on modern art, her own ghastly daubs on the wall - like swirling doughnuts and bloated owls' eyes - demolishing her argument before she's made it.

Both couples have come out the other side of their own sex lives. Dawn, a dental practice receptionist in a very tight, orange two-piece outfit, is not averse to shopping around, though. Alan's dangerously obese and swilling beers in his England football jersey. His baby is Vince; Dawn's, from another relationship, is on tour with the army in Afghanistan.

Oliver has re-joined the Labour Party, whose credibility, Emily thinks, "went up in smoke the day they buried John Smith." Her knee-jerk dislike of Tony Blair's war-mongering runs unanswerably into the pride and feeling of the parents of boys who join up to serve their country.

This clash is presented, without apology in Ellie Jones' fresh and cleverly acted production, as further evidence of a real north/south divide. On a more mundane level, Oliver and Emily have invited their opposite numbers in for drinks having foresworn alcohol; Alan brings his own tinnies. Their resolution cracks, though, and a bottle of wine is eventually produced from beneath an Indonesian mask.

Three upheavals - Oliver's mum's financial legacy; a cataclysmic cat crisis; and a sexual revelation - threaten to prompt a swift return down south to "Planet Earth". But not before a fourth piece of news prompts the most startling transformation of all, and the dark day of a new Dawn.

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