There are two remarkable performances at the heart of Alan Ayckbourn’s latest dark comedy. Alexandra Mathie and Matthew Cottle play the middle-aged brother and sister around whom the action of Neighbourhood Watch revolves. As the story develops and the tension heightens, the different yet oddly parallel characters of Hilda and Martin twist up out of near-caricature into something truly dangerous.

Pip Leckenby gives us a set which is as odd as the people who inhabit it – dark walls, two openings, two sofas and a central fire feature. Ayckbourn himself directs and does far better by the second act than the first. This introduces us to some of the neighbours in the Bluebell Hill development, to which Hilda and Martin (committed Christians both) have just moved. They’re as daft a bunch as you’d meet anywhere anytime in Ayckbournland, locked in unhappy marriages and seething under whole shed-loads of frustrations and unrealised ambitions.

Magda (Amy Loughton) is a little mouse of a music teacher, verbally and physically abused by her husband Luther (Phil Cheadle). Amy (Frances Grey) with her dyed hair, skimpy tops and deliriously short skirts is making up for the inadequacies of her engineer husband Gareth (Richard Derrington) – who it transpires has a nice sideline in medieval methods of punishment – spreads her favours indiscriminately.

Then there’s Dorothy (Eileen Battye), now retired from the advertising department of the local newspaper, and her friend Rod (Terence Booth), an ex-policeman and former security guard, yearn for the days of proper law-and-order and respect from the young for their elders. Small chance of that, of course. So a Neighbourhood Watch scheme is started, initially tentatively with only a few supporters and bland indifference from the police.

Somehow this mushrooms from basic security measures, such as individual high fences, to a razor-wire perimeter with a manned gate, identity cards, baseball-bat armed patrols (courtesy of one of the local criminal families) and a nasty whiff of the sort of police state which monitors morals as well as manners. Good as Cottle’s Nuremburg rally-style speech is at the climax of the first half, Mathie’s penultimate scene towards the end of the play is even more chilling.

Not nice people really, any of them. For me the trouble with the first hour is that it’s a little too obvious that these Daily Mail readers (this reference and a subsequent one to The Guardian earned the loudest laughter from the Cambridge first night audience) are being set up like ninepins – or garden gnomes – to be knocked down. They don’t become actual human-beings who suffer just as much as they appear ridiculous until a bit too late in the action. But then they come into their own. And take us over.