ALAN Ayckbourn recalls the spark that ignited Joking Apart, his 1978 play that opens with fireworks. "Why don't you," a woman once asked him angrily, "ever write a play about a happy couple?"

Taking up the gauntlet, he came up with Richard and Anthea, both separated from first marital partners and now living in happy unison, but no legal union, as the impossibly golden couple of everyone's dreams in the Old Vicarage, somewhere in middle England.

So here was his happy, carelessly charming, business savvy pair, but inevitably a sting awaited in the Ayckbourn tale: the disastrous, unhappy effect they had on their friends, business colleagues and neighbours.

"Joking Apart was the nearest I’d got at that stage of my life to writing an autumnal play, about the sadness of getting older," says Ayckbourn of an outdoor drama that spans 1966 to 1978.

Since its January 1978 debut, the play has taken on the landmark status of being the one where darkness settled on the playwright's landscape of middle-class suburbia: hence that Joking Apart title.

That status is an over-simplification, but this garden comedy has grown into "one of my favourite plays", says Ayckbourn, who is directing it for the fifth time in its 40th anniversary year, and the first time since 2002 at the SJT (when York Theatre Royal pantomime villain David Leonard played Richard in lounging mode incidentally).

We join athletic, handsome, Midas-touch businessman Richard (Laurence Pears) on Bonfire Night 1966 in the unkempt but lovely Old Vicarage garden (designed by Michael Holt). He is lighting fireworks - a metaphor for his impact on those around him - while pretty, perfect-hostess partner Anthea (York actress Frances Marshall) is dispensing sausages and drinks.

They have invited their new neighbours, timid vicar Hugh (Jamie Baughan) and hesitant wife Louise (Louise Shuttleworth); business partner Sven (the outstanding Leigh Symonds), a sceptical, always right, apoplectic Scandinavian, and his weight-obsessed, put-upon but bubbly wife, Olive (Liz Jadav); and Richard's friend Brian (Richard Stacey) with the latest of his strange, awkward, unsuitable girlfriends (Naomi Petersen).

Over four exquisitely played scenes, at four-yearly intervals over 12 years, Ayckbourn and his typically wonderful cast portray the devastating, insidious consequences of befriending the unwitting Richard and Anthea, as health and relationships crash around their surfeit of generosity amid tennis on Boxing Day, croquet on a summer's day, and food and drink, whenever, wherever.

The creeping sadness, amid the dripping tap of comic truth, is best summed up by Sven's assertion: "The tragedy of life is not that man loses, but that he almost wins." Classic Ayckbourn.

Joking Apart, Stephen Joseph Theatre, Scarborough, in rep until October 4. Box office: 01723 370541 or at sjt.uk.com