Ever since his sensational biography of Peter Sellers was first published in 1994, Roger Lewis has been haunted by a guilty feeling that he might have killed Sellers’s fourth and final wife, the model and actress Lynne Frederick.

Lynne died on the eve of the book’s publication. That week, newspapers had been previewing titbits, including his portrayal of her ‘fatal marriage’ to Sellers.

‘Of all his wives,’ Lewis wrote, ‘Lynne had the most consistently terrible time . . . he treated her like a retainer.’

Nor, he wrote, was Sellers in good physical shape by the time he married Lynne in 1977 (he was 52 with a dodgy heart; she was 22).

The way Lewis describes Sellers’s lost sexual prowess is typical of his merciless writing.

‘He was on so many drugs that his valves were popping open and shutting like the mouth of a goldfish, and the hydraulics in his loins clogged and wilted.’

Lewis was nervous that Lynne, on reading this ‘insult to Sellers’s memory’, might ‘unleash the dogs of war’.

Instead, she died – although quite how has never been officially determined. Lewis is clear that she choked while eating meatballs and spaghetti; her mother meanwhile claimed she’d died in her sleep from a seizure.

The autopsy report failed to determine a cause of death but did rule out suicide or foul play.

Lewis surmises that if his book did contribute to her exit, it was ‘only because I’d written what she knew to be the truth, and how could she bear the truth?’

In this 100th anniversary year of Sellers’s birth, Lewis’s biography is being reissued, with a new chapter called Centennial Sellers, swelling it to 920 pages (excluding the index) – which would be unforgivably long, were it not for Lewis’s consistently engrossing prose.

Now Lynne is dead, Lewis really lays into her. He describes how she became tainted by Sellers’s own vileness. ‘To appease him, she grew like him . . . she was pulled towards the abyss at Sellers’s core.’

She became ‘a hypocrite, a nymphomaniac, a hysteric’, as well as addicted to vodka and pills: after his death ‘she squandered £2,500 a week on nose-candy’, ‘her slender figure ballooned to 14 stone’, and she was ‘callous’ to Sellers’s children by his previous wives, refusing them access to his photos of them.

In the new chapter, Lewis describes being on set during the making of the 2004 film The Life And Death Of Peter Sellers, based on his book, with Geoffrey Rush as Sellers, Miriam Margolyes as Sellers’s mother Peg and Charlize Theron as Sellers’s second wife the Swedish Bond girl Britt Ekland (who also had a hellish time being married to Sellers – during one tantrum on holiday in Ischia, Italy, he upended a bed and broke her front tooth).

Lewis prayed the film wouldn’t end up as a ‘straight-to video turkey’. It didn’t. Rush won a Golden Globe, and the film won nine Emmys.

The book hammers home, with countless examples, what a vicious, selfish, spoilt, jealous, mean-minded man Sellers was, and how, the richer and more successful he became (especially after his success as Inspector Clouseau in the Pink Panther films), the more impossible he was to work or live with.

‘He was more interested in making trouble than making a film,’ Lewis writes, describing how he refused to do re-takes, failed to turn up or learn his lines and got underlings he didn’t like sacked.’

Kingsley Amis called him ‘one of the most self-satisfied s***s I have ever met.’ Lewis goes further. ‘Sellers was evil. He was a destroyer. He was pandemonium.’

He was a terrifyingly unpredictable father: ‘like a playground bully’, his son Michael said. ‘He enjoyed making you cry, and once his victim had started to break down, he’d press his advantage.’

He was physically violent with his first wife Anne, breaking furniture, ripping radiators from walls, smashing porcelain and snapping her pearls. Once, he locked her in a cupboard, only letting her out on the condition that she promised never to look at another man again in her entire life.

The source of his spoiltness was his relationship with his mother Peg, who smothered him with adulation during his roaming childhood in the touring theatre business.

The sycophancy he demanded and received in adulthood (for example, he was given Rolls-Royces as inducements to read a script) was ‘an increase in the glutinous veneration he’d fed off under Peg’.

His two most obsessive, all-consuming crushes were on two women who didn’t return his lovesickness. The first was the actress Sophia Loren. He told Anne he was in love with her, and their marriage started falling apart from that moment on.

‘It was total madness, the way he kept blabbing to me about Loren,’ Anne said. ‘He’d lie in bed and say, “I can feel her spirit coming into the room.” ’

By being unattainable, Loren was forever on a pedestal for him, unlike the women who did succumb to his seductions, whom he quickly grew to despise.

He was also consumed by love for Princess Margaret, when she was married to Tony Snowdon. He believed she would marry him, and sent her a stream of romantic letters. She wore the same bra size as Sophia Loren, he discovered: ‘what further sign and portent did the world need?’

So he was furious one day when, on the set of the 1967 satirical version of Casino Royale, Princess Margaret appeared, walked straight past him, and said to fellow actor Orson Welles, ‘Hello, Orson, I haven’t seen you for days!’

Sellers refused to act with Welles again and walked out of the film, causing mayhem to the production. Was he a genius? Devotees of the 1950s radio show The Goon Show think so.

Lewis writes that ‘his genius lay in being able to produce any one of a thousand convincing voices from up his magician’s sleeve’. He was excellent in The Ladykillers, Dr. Strangelove and Being There. But he was in some bad films, too, and Lewis calls him ‘the great actor who was seldom in anything good’.

With a casual flick of his prose at the end of the new add-on chapter, Lewis changes his tune. ‘When I was writing the book 35 years ago, with a wife, children and domestic responsibilities, the horror of turning his back on all that, as Sellers did with Anne, struck me forcibly.’

Now, he feels, ‘irrational and unstable he may have been, but he was funny. Who cares if he was a b****** off-screen?’

Reading this account of Sellers’s life of consistently callous behaviour, I think there’ll be a great many who do care that he was vile.