‘Ready?’ he asks, before penetration. ‘As I’ll ever be,’ she replies

“I get lack of desire a lot. Lack of passion. How do I get it back?” So says Joy, a therapist meeting two prospective clients. “Generally speaking, we are very bad in this country at discussing our private lives.”

This is a more astute comment than even she realises, for in just a few seconds Joy has not only diagnosed the problems of the couple before her, in the death throes of their relationship, but also the titillating, fidgety attraction of that great British institution, the sex comedy. If you can’t discuss something kept rigidly out of view, you can at least giggle when it becomes hopelessly exposed.

In Wanderlust (BBC One, Tuesday, 9pm), Nick Payne's disarmingly funny television adaptation of his 2010 play, sex is an awkward affair. Physician, heal thyself, you might tell Joy, magnificently played by Toni Collette, for whom marital desire and passion have been replaced with irony and snark.

“Ready?” her husband Alan asks, in the show’s opening seconds, just before penetration. “As I’ll ever be,” she replies. The earth, I’m afraid, does not move.

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There’s some reason for all this bracing: the scene is intercut with jagged flashbacks to Joy’s recent cycling accident – not serious, but significant enough to have put her out of the saddle.

Still, nobody gets a smooth ride in Wanderlust. Two characters are discovered masturbating, at work and at home, spurring awkward conversations and, by and by, sadder confessions. The school crushes of Joy's son are adorably, inevitably tangled. Her older daughter's relationship fizzles out.

And ego-boosting flirtations become bolder – between Alan and a colleague, and between Joy and a man at her local swimming pool – until they become outright betrayals.

Like one joke, a grace note involving their neighbour’s unspeakably awful cannelloni, characters would prefer to leave things unsaid than make a scene. That suspends them, like the exquisitely pathetic neighbour Neil, between comedy and tragedy, which director Luke Snellin’s accentuates with sly angles, lonely frames and some jolting editing that punctuates dialogue with wordless images.

In one marvellously funny, transgressive scene, archly filmed from the hips, Joy and her admirer Marvin make panicked but polite conversation during a frenzy of heavy petting, as though they were meeting at a dinner party. “I don’t know what you do?” she says, apologetically. “I work for the constabulary,” he says. “No!”

This presages a new kind of frankness between the married couple, when Alan admits to sleeping with his colleague and learns, in return, about the manual sex of Joy: “I tossed off a man from my hydrotherapy group.”

Agreeing to open up their marriage to others, it’s the most direct and intimate exchange we’ve seen between the couple, and the camera finally brings them face-to-face, with each other and with us. So begins the darkly comic come on of the series: how much honesty can two people take?