The David Attenborough of bourgeois London, Nigel Williams has spent his writing career peering through a telescope or clambering over hedges to get a better glimpse of the peculiar mating habits of the species who inhabit SW19, Putney, and similarly leafy urban environments.

A former BBC man who writes for radio and was feted for his film script for Elizabeth I, starring Helen Mirren, his fiction, more than most, has a distinctive voice. In fact, it's usually a case of several voices, because years in broadcasting have given Williams a sharp ear for character as revealed through speech. Perhaps understandably, given the success he had with the Virgin Queen, he seems particularly drawn to chilly, difficult but fascinating women.

In Unfaithfully Yours, an epistolary novel written by nine different hands, he acknowledges that letters are as dated as Egyptian mummies, and almost gleefully flouts all rules of the contemporary novel by slowing its pace to the Royal Mail's speed. In so doing he allows each of his cast to stand centre stage without interruption, as if they'd just grabbed the mike. Thus reading this novel feels strangely, but not surprisingly, like listening to a radio play.

William's slow-burn device is a pleasure, and the first of many in this rambunctious and rather magnificent piece of tomfoolery. But lest anyone think this is nothing more than a farrago of nonsense, which in large part it is, the author's clownish plot is infused with poignant observations about love and its perils to which those who, like his characters, are in late middle age will surely warm.

Set in Putney, where Williams lives (and Elizabeth I often visited), Unfaithfully Yours begins with a letter received by private detective Orlando Gibbons. It's from a starchy classics teacher, Mrs Elizabeth Price, who wants him to find out if, or more likely with whom, her husband is being unfaithful. This dispirited spouse is sure of only one thing – that the priapic barrister is not a paedophile. "We have two children," she writes, "and, as far as I know, he has never interfered with either of them. Indeed, it has been something of a struggle to get him to even acknowledge their existence."

As Gibbons gets into his 'stride' – he has an unquenchable love of inverted commas – he finds himself drawn into the affairs, in every sense, of four couples who have been friends for decades. A discordant crew comprising a doctor, dentist, novelist and the like, in earlier times they holidayed together in the Costa del Sol, hence Gibbons dubbing them "the Puerto Banus Eight".

What ensues is a romp, or a rammy, as partners reveal the miseries of their marriages, find new love, or discover they are gay (sometimes all three). The joy with which Williams evokes Mike, the suddenly homosexual documentary maker, whose speciality is the gudgeon, and who, in his newly liberated form, intends to record the gay lives of the animal kingdom, is the sunniest part of the book. "I left the BBC nine years ago, driven out by John Birt," bemoans Mike, " – never a man who understood wild animals since they showed a remarkable lack of interest in focus groups..." Least happy is the mystery over Mike's late wife's supposed suicide, 10 years earlier, which is rumoured to have been murder.

Plot strands that at first were neat as chopsticks quickly come to resemble cooked spaghetti. A master plasterer, slapping on plot and farce with a heavy hand, Williams relishes the ridiculous. At times he seems almost to wink to the reader as if to ask, how much more of this can you take?

The answer is, more than you'd think. Williams's tone can be music-hall, his humour crude, his characters caricatures. Yet – and it is a significant caveat – he is a consummate entertainer. As with all good comedy, this tale is underlain by a vein of sadness and regret, of belated self-knowledge and understanding. This serves to explain, and even excuse, the decidedly odd behaviour not just of his characters but, when you think of it, of us all.

Unfaithfully Yours

Nigel Williams

Corsair, £18.99