Middle-aged brother and sister Hilda and Martin move onto the Bluebell Hill Development bubbling with excitement: the house is perfect, the view is charming and the neighbours are sure to be a decent bunch. Eager to make a good impression, they host a house-warming. Out of over 100 invitations, only a handful of people turn up, but this does nothing to dampen the siblings' expectations of their new life in Bluebell Hill. What does rattle them, however, is their new neighbours' insistence on the need for increased security. Hilda and Martin try to laugh these warnings off, but when Martin is attacked in broad daylight by a trespassing youth, their neighbours' fears appear to be confirmed. Something will have to be done.

Bluebell Hill's collection of misfits are little more than stereotypes – the meek bachelor, the busy-body neighbour, the security-obsessed former soldier – but they're amusingly drawn. As the group puts its plans for a neighbourhood watch scheme into action, playwright and director Alan Ayckbourn's eye for detail and flair for dialogue make for an enjoyable first half. It's like watching a re-run of an old-fashioned television sitcom: you get the sense of having seen it before but the chuckles come frequently enough to stop you from changing the channel.

It is not long before the interval, however, when the play begins to lose its way. Flushed with the success of their initial security measures, the members of the neighbourhood watch committee adopt an increasingly draconian approach. Ayckbourn's parody of the paranoia of Middle England in the face of the disintegration of public morality isn't sophisticated or new, but it's entertaining nonetheless.

As the play goes on, however, rather than embracing the spirit of farce engendered by the twists of the ever more far-fetched plot, Ayckborn bottles it. Various issues are thrown into the mix. References to domestic violence, child abuse and latent homosexuality – all presented with a veneer of serious drama – clash with the increasingly surreal nature of the piece, leaving us with a sense that this play doesn't know quite what it wants to be.

The cast do the best with what they've been given – Eileen Battye as gossip-monger Dorothy deserves special mention – but ultimately too little attention has been paid to how these characters interact with one another, with the result that barely any of the play's relationships ring true. Frances Grey, who plays Amy, the one-dimensional neighbourhood adulteress, has been given a particularly raw deal by Ayckbourn, in terms of both character and direction. Her behaviour is unbelievable, her motivations unexamined.

This slight, safe piece – the playwright's 75th – may please Ayckbourn afficionados, but it's hard to imagine anyone else getting much out of it. Why Nicolas Kent has chosen to round off 27 years as artistic director of one of the country's most influential political theatres with it is anyone's guess.