There’s a story Stanley Kubrick’s friends like to tell. On the set of Eyes Wide Shut, the renowned perfectionist had outdone himself.

In a scene where Sydney Pollack’s wealthy socialite addresses Tom Cruise’s doctor, the actors had been requested to go through the motions yet again. Pollack had been hired for three days’ work, but his tenure had already extended into a third week. They had performed the scene hundreds of times, covering every conceivable interpretation. Why on earth, Pollack asked, would the director possibly need it repeated? Quipped Kubrick: “Don’t you want to get it right?”

Jan Harlan laughs when recounting the tale. As Kubrick’s brother-in-law, the sibling of Kubrick’s widow, Christiane (both German-born), he knew the man, who died of a heart attack in March 1999, better than most. His documentary Stanley Kubrick: A Life in Pictures stands as the best of the filmed tributes.

In the late 1960s, as a businessman, Harlan had been brought in to negotiate the rights to Arthur Schnitzler’s Traumnovelle, the book on which Eyes Wide Shut was based. He ended up as executive producer on all Kubrick’s films from Barry Lyndon (1975) onward. “I didn’t know about films, other than going to the movies,” the 73-year-old shrugs. Such was Kubrick’s universe that those who served him well got sucked into his orbit, unable, usually unwilling, to resist.

Another such was the film-maker Andrew Birkin, who began his career as a tea boy on 2001: A Space Odyssey. He demonstrated some precocious ingenuity in the art department, so Kubrick promoted him to “first assistant director, special effects”. Both men were to play key roles in a Kubrick project that has been hidden from public view for 40 years.

In recent times, there has been as much interest in the movies Kubrick didn’t make as those he did. His sci-fi film AI, conceived before the computer effects existed, was bequeathed to Steven Spielberg. The Aryan Papers, a film about the Holocaust that Kubrick, a Jew, had planned, was abandoned when it ran into competition with Schindler’s List.

Most intriguing of all, though, is Napoleon. A purported three-hour epic, it would, at a budget of $5.2m (about $100m today), have been the most expensive film of its time, shot in the UK, France and Romania. For 40 years, its content has been the subject of conjecture among film buffs; snippets of information leaked out about this unrealised “masterpiece”, but the critical material stayed under lock and key.

Just north of St Albans, in Hertfordshire, nestling in rolling countryside, the 120-acre Childwickbury estate is reason enough why Kubrick remained in England, making all his films here from 1962’s Lolita. He bought Childwickbury Manor as a leaky pile in 1978, its restoration a labour of love.

Today, in the hallway, the sinister masks from Eyes Wide Shut leer from the wall. Off the atrium, in which Christiane’s brightly coloured paintings hang, is the library, once a private screening room. Around the cavernous kitchen flit family members, still in residence, and cheery household staff. It was in the kitchen that Cruise and Nicole Kidman rehearsed. The big oak table is the one at which Jack Nicholson hammered out his “All work and no play” manuscript in The Shining, a prop from Pinewood. Cruise once landed his helicopter on the lawn, Harlan says. It frightened the dogs. (They roam freely, accorded the status of Hindu cows.) And there, across the grass, within a shady copse, rests the man himself, his presence still looming.

An outbuilding is now an Aladdin’s cave of boxes and flight cases, housing material from all his films. Original prints — labels reading “The Killing Reels 1-5”, “The Shining Reels 6-9” — lie jumbled. Harlan opens a safe and pulls out the heavy Zeiss lens, of Nasa specification, that enabled Kubrick to film scenes of Barry Lyndon by natural candlelight, one of numerous technical breakthroughs. And there, tantalisingly, sit those boxes marked “Napoleon”.

Some of their contents finally see the light of day this month, with the publication of a set of books entitled Stanley Kubrick’s Napoleon: The Greatest Movie Never Made, from Taschen. The title is a play on Kubrick’s own boast when touting the project for an intended 1972 or 1973 release. “Making a book about an unmade film is quite an unusual challenge,” says its editor, Alison Castle. The boxed set of volumes, featuring Kubrick’s 28,000-word “treatment”, transcripts of conversations with academics, the annotated screenplay, even pictures of finished prototype costumes, as well as sundry research materials, is a collation culled from 88 boxes — no mean feat, judging by the thousands of documents I saw exploding from one trunk alone.

Napoleon once remarked that his life would have “made a great novel”. “I’m certain he would have said ‘film’ if movies had existed at the time,” Kubrick mused. Though, when he referred to the Frenchman’s “life”, he meant it literally, his stated aim being to re-create the whole of Bonaparte’s journey from Corsican ruffian to St Helena exile, taking in the French revolution, the entire Napoleonic war and his equally tumultuous love affair with Joséphine de Beauharnais. Spanning half a century, Kubrick’s movie was to be set on a broad inter national canvas — Lisbon to Moscow, Channel to Med. David Hemmings was his nominal “Boney”.

Napoleon had been done before. There was Abel Gance’s monumental silent version; Marlon Brando had mumbled through 1954’s Désirée. Kubrick’s take on Bonaparte was different again — not the standard British one, that of the proto-Hitler given his comeuppance by Wellington, but more the ambivalent French assessment: of a quasi-socialist liberator whose lust for glory went to his head. This was to be no “dusty historical pageant”, Kubrick assured. “It has everything a good story should have. A towering hero, powerful enemies, armed combat, a tragic love story, loyal and treacherous friends, and plenty of bravery, cruelty and sex.”

In 1967, at MGM, which then had a studio at Borehamwood, pre-production began. Birkin was presented with the opportunity not just to be involved, but to be integral, as Kubrick’s assistant director. On 2001, he had helped to advance the process of “front projection”, the means by which pin-sharp still images could be shone onto a screen behind the actors, creating a photorealistic backdrop, most evident in the desert canvas for the Dawn of Man “Apes” scene at the beginning of the movie.

For Napoleon, Kubrick intended to employ the technique again. “He got rather carried away with the idea when it came to Napoleon — ‘Gee, I could actually shoot the greater part of the film in these actual locations without having to go to the actual location,’” Birkin says. “Partly because he didn’t like travelling, and partly because they wouldn’t have let you shoot in places like Versailles and Fontainebleau.” With 2001 out, Kubrick’s stock was high. The French minister of culture gave Birkin carte blanche to scout and photograph locations as he pleased. Charged by Kubrick with the mission “to go everywhere Napoleon ever went and photograph it”, he set off in June 1968. Despite the distractions of Paris — his actress sister Jane had struck up a relationship with Serge Gainsbourg, not to mention the student riots — he started amassing the photographic references.

Back home, a team of Oxford graduates was toiling under Dr Felix Markham, whose 1963 biography of Napoleon was Kubrick’s bible. Meanwhile, Harlan was dispatched to Romania to negotiate the use of the army — 10,000 cavalry and 40,000 infantry — for use in the set-piece battle scenes, notably Austerlitz and the retreat from Moscow. They hit upon an ingenious cost-effective method. For background extras, military uniforms could be printed on a new “rip-proof” paper being trialled for the envelopes of FedEx.

Within months, MGM had bankrolled pre-production to the tune of $420,000. Further actors were mentioned — Peter O’Toole, Alec Guinness, Charlotte Rampling, Jean-Paul Belmondo. And, although Audrey Hepburn turned down the role of Joséphine — “She wrote him a letter saying she didn’t want to do it, she didn’t want the trouble,” Harlan says — things were shaping up for a scheduled shoot over 1970-71.

Not for long. “I got the message when I was in Vienna. ‘Come back,’” Birkin says. He had got Kubrick a Christmas present, a copy of Napoleon’s death mask. “He opened it, and he was a bit unsettled… this thing staring up at him. He almost went white. He said, ‘Have you heard the news? MGM have pulled the plug.’” A change of leadership and an economic downturn had given the studio the jitters. The recent flops Hello, Dolly! and Star! had suddenly made costume romps a high-risk venture. In a statement of January 1969, Napoleon was officially kiboshed.

To boot, the Russian director of War and Peace, Sergei Bondarchuk, had begun shooting Waterloo, with Rod Steiger as the diminutive Corsican. By the time Kubrick had taken his project to United Artists, that, too, had proved a box-office failure. There was no appetite. In need of a quick fix — quick for Kubrick — he sought something he could make relatively simply. He turned to a book by Anthony Burgess, A Clockwork Orange.

Napoleon did raise his head again after that, with the enticing prospect of Jack Nicholson in the lead, an actor Kubrick was destined to work with. But it was never a serious proposition, Harlan insists, just an attempt to attach some star wattage. And so the project lapsed, some of its know-how recycled into the lavish Georgian drama Barry Lyndon. What was left was an enormous Napoleonic archive, possibly the largest private one in existence. Harlan opens a file drawer containing a section of the 17,000 or more images — photographs of paintings, mock-ups of uniforms, buildings — all assembled on IBM punch cards, which were intended to be sorted by a Hallike computer, a cutting-edge precursor of the database.

In the end, the research became an end unto itself — “Stanley liked soil samples of the battlefields,” Birkin says — with the director, who was not averse to a bit of erotica (he had wanted to make a spoof porn film), showing an eager curiosity as to the sexual habits of the bourgeoisie. Was Joséphine “part of a swinging set?”, he asked Markham. Would it be “a smear on Joséphine” to portray her thus? (Clearly not: in Kubrick’s script, Napoleon meets his beloved at an orgy.) “I know from endless conversations with him that what really interested him in Napoleon was his human frailty,” Harlan adds. “Kubrick always said that’s what interested him in humanity, that we are not governed by our intellect. In the end, we are governed by our emotions.”

So, how good would Kubrick’s Napoleon have been? Having proved his mastery of spectacle with Paths of Glory and Spartacus, there’s no doubt he had the right stuff. Trimming it all down to a palatable running time would have been a tall order (a script dated September 1969 logs the film at 200 minutes). In 1973, the BBC had mounted a 20-part serialisation of War & Peace. “Stanley had to admit, ‘Actually, that’s the way to do Napoleon,’” Harlan recalls. “‘Don’t limit yourself to three hours — make it 10 hours, make it 12.’”

Harlan says that we shouldn’t set too much store by the official screenplay: “It was really written for film executives to read. The film would have been much more subtle, much more complex.” But the movie would have packed quite a punch. “He would have got inside the mind of Napoleon,” Birkin says. “You have to be something of a dictator, a great dictator, to let an audience empathise with Napoleon… who was quite like Stanley in his own way.

“I think Stanley’s script about Napoleon says as much about Stanley as it does about Napoleon. As a piece of cinema, it would have been dazzling. Whether it would have made any money, God only knows.”

In the script, as in Barry Lyndon, a coolly ironic voiceover links the key events in Napoleon’s life. It is this, says Birkin — presenting the story as “Napoleon’s greatest hits” — that might have been its undoing. “Stanley was trying to tell the story in three hours, which is virtually impossible if you’re telling it from the cradle to the grave. You’ve either got to take a section of it or you’ve got to find a rather stylised way of doing it.”

Napoleon may ride again. “It’s a film. It’s available,” Harlan says, pointing out that MGM still owns the rights. “I had a meeting with Steven Spielberg and Ang Lee, the three of us together. We all agreed Ang Lee would be a great director for it, but he was committed to Hulk and it wasn’t picked up again. We came very near.”

If the right person were found and the trustees approved, the estate would gladly turn over the research material. “But it needs somebody with tremendous clout, a big director. You don’t raise $100m easily, not on such a risky picture. Ridley Scott, he has the script. If Ridley Scott wanted to do it, he could raise the money, and I would applaud him.”

Would Kubrick have wanted it made? In the end, even he had misgivings. In a letter to the military historian David Chandler, dated July 12, 1976 — a piece of correspondence absent from the Taschen collection — he pointed to his “concerns”, reluctantly drawing his own conclusions as to the content of his beloved project. “Namely that the thing seems jumbled together and stuffed into a [small] box…”