A WHILE back, I was rebuked by Graham Swift for referring to his latest work as a novella. Mothering Sunday was not a novella, he said, it was a novel. It had the arc of a novel, and the richness of content, and was written to provide the satisfactions the novel offers, in ambition, story and style. It was slim, certainly, but to refer to it in this way, he intimated – in front, I should add, of an audience of 600 or so – was to belittle it. Not that he used that word. And since that day, I have not used the word novella either.

How right he was. When I think of my favourite books, several of them are short or very short novels, Mothering Sunday among them. The more fiction you read, the more you learn that slim does not mean slight. I first learned this in a very practical way, when asked to abridge various books for BBC Radio 4. Faced with a choice of which to reduce drastically into five or 10 quarter-of-an-hour broadcasts – a 500-page thriller, or a story that fills 150-pages – you'd think the answer would be easy. It must be far harder to cut down a fat work than a thin. But it's not. Writers of skinny books have already done the cutting. Good writers, that is. Almost everything on the page is essential to the story. Unpick one strand, slice off a scene or a conversation, and the tale is left like a house hit by an earthquake, a supporting beam fallen, a chair with a leg dangling precariously over the void, the whole structure unstable and broken.

Of all the abridgements I've done, one of the most fiendish was Michel Faber's The Courage Consort. It is a work small enough to hide behind your mobile phone, but so intense, every line filled with meaning, that taking the scalpel to it felt like an act of vandalism.

Bernard MacLaverty's Grace Notes, though longer, was the same. Impeccably written and modulated, it made the abridger's task a recipe for insomnia and regret. I never could understand a friend who boasted that he had improved every book he ever abridged.

Most recently, however, I was faced with probably the toughest of all: Muriel Spark's A Far Cry From Kensington. The second abridgement in the BBC's on-going The Vital Spark series celebrating the centenary of Spark's birth in February 1918, it has been more difficult, in part because many listeners will already be familiar with the book. As you set about deciding what to remove, you know you will be excising favourite passages and lines – you know this because they are yours too. But even if this tragi-comedy of post-war London literary life were completely unknown, it would be a daunting task. I had 10 episodes to play with, but what at first seemed like luxury, soon proved the bare minimum required. Although the novel has fewer than 60,000 words, each episode is 2,000. That means cutting the book by around two-thirds.

It's an appropriate fraction, given that in this novel we meet Spark's heroine, Mrs Hawkins, a war widow in her late 20s who is enormously fat. Part-way through the novel she has a revelation and decides drastically to reduce her size. She uses the simple, commonsensical and effective method of eating and drinking half of what she is offered, or would usually have eaten. "After a while," she advises those who might like to follow her lead, "if you are a perfectionist, you can consume half of that again."

Dropping dress sizes carries a metaphorical weight in this sophisticated, witty and at times profoundly sad story. It can also be cuttingly satirical. The story of Mrs Hawkins, who lives in a cheerful boarding house in Kensington and works in publishing, it is a tale of deception, gullibility, and the frightening power of religious and superstitious faith. The victim is not Mrs Hawkins, but one of her housemates.

The story is also a romance, but even more memorably, it is a revenge tale, taken by the author on her former lover, Derek Stanford, who years earlier had betrayed her trust by stealing and selling some of her correspondence, and later writing about her in unflattering terms. He is reincarnated in the intolerable Hector Bartlett, a dreadful scribbler whom Mrs Hawkins calls a pisseur de copie – who "vomited literary matter, he urinated and sweated it, he excreted it". Unfortunately, she does not keep this harsh but accurate description to herself but throws it in his face, thereby starting a sinister series of events that ends terribly. Narrated in hindsight by Mrs Hawkins some 30 years later, A Far Cry From Kensington flits between the early 1950s and the time of the book's publication, 1988, when Nancy, as Mrs Hawkins is now known, is recalling every detail as she lies awake, sleepless, a 'beloved' habit that has never left her.

You can read a Spark novel on several levels, and enjoy every one of them. One of the aspects that I relish most are her characters, people created in a dash of the pen, as if by an artist's brush. One minute the page is blank, the next a fully formed individual has taken to the stage and is hogging the limelight. It was one of these who was the first and deeply regretted casualty of this abridgement. Next door to the boarding house in Kensington lived a friendly Cypriot mechanic and his partner, with whom he would often have rows. On one particular night the commotion was so bad that Mrs Hawkins and her landlady Milly sat on the stairs, under cover of darkness, passing around a box of chocolates as they watched the pair shouting and thumping each other next door. Finally they summon a policeman. "'Can’t you stop them?" said Milly, passing the chocolates.

"The policeman accepted a chocolate. ‘Mustn’t come between husband and wife,’ he said. ‘Inadvisable. You get no thanks, and they both turn on you.’"

It feels like ripping out some of the joy of a book to remove a scene that, while not strictly essential to the plot, is part of the supporting structure that adds texture, evoking the randomness of reality and, in Spark's case, comedy. But go it had to, along with countless other zingers.

Reading a book with an eye to what can – or must – go is a peculiar and enlightening process. You are reading with an X-ray eye, and the result is clinical: you see how the book is composed and built, how scenes dress the bare boards and mask the joins, and how the dancing between past and present is artfully deployed, to break and enhance the tension and mood. By the end, my awe of Spark, and this, one of my favourite of her novels, was even greater than before.

Of course, at first, the winnowing process is troubling. Guilt is the predominant feeling. Gradually, however, a fresh story emerges, one that suits its new medium. What initially felt like a brutal reduction no longer seems so savage. What had seemed pared to the core, mysteriously regains shape and flavour. And what emerges, as strongly as if reading the book, as powerfully as if it had first been written this way, is the author's voice, and genius.

A Far Cry From Kensington, read by Maggie Service, abridged by Rosemary Goring and produced by Eilidh McCreadie, begins tomorrow on BBC Radio 4's Book at Bedtime, at 10.45pm. For more information on the BBC's The Vital Spark series, go to bbc.co.uk/programmes