Napoleon is a silent film directed by Abel Gance, dramatising the youth and early career of Napoleon Bonaparte. Its most complete screening, said to be nine hours long, took place in Paris in 1927 – but this version was subsequently lost. British film-maker Kevin Brownlow saw a version as a schoolboy and subsequently restored the film to close to its original length from various prints. His restoration was first shown in London in 1980 with a score by Carl Davis. It will screen again on 30 November at the city's Royal Festival Hall.

Kevin Brownlow, restorer

It was 1953 and I was still at school. I'd borrowed a silent French film from the library for my 9.5mm projector. It was by Jean Epstein and it was awful. So I rang the library and asked if they had anything else. They said they had Napoleon Bonaparte and the French Revolution. "Oh that will just be a classroom film," I said, "full of engravings and titles and all very static."

But when it arrived, we played it on the wall and I'd never seen anything like it. This, I thought, is what cinema ought to be. I realised what I had was two reels of a six-reel version put out for home cinema use. So I started advertising in Exchange and Mart until I got the rest of it. And then people started coming to see it.

I wrote a letter to the director, Abel Gance, care of the Cinémathèque Française in Paris, because I couldn't believe what I'd seen. He answered it – which was very, very unusual. And then one day he just walked into the British Film Institute. James Quinn, who was in charge, knew of my interest and rang my mother. She called my school. I was in the middle of a mock exam, but they let me go to see him.

Some directors were brilliant in the silent era but never felt at home in sound. It's like a sculptor being forced to take up painting. I remember Gance saying he felt he'd made all of his sound films with his eyes shut. He was so friendly, so surprised to find this little kid who was besotted with his work. I think we were on our own for a lot of the time. The only drawback was his lack of English, although enthusiasm doesn't really need translation. It couldn't have been better. If I had done the same with somebody like Fritz Lang, I'd probably still be nursing the wounds!

I've always thought Gance panicked as he neared the end of the film, realising the audience would expect Waterloo, even though this final defeat occurred late in Napoleon's life. Gance felt he had to come up with something else, an idea that would leave them stunned. That was the triptych – when the screen opens out and becomes three times as wide. This requires three projectors. Nobody has ever said: "Where's Waterloo?" The ending, which I'm not going to give away, is so brilliant it just takes your breath away.

I decided to restore Napoleon after a widescreen festival at the Odeon Leicester Square in 1968. It was run by Richard Arnell and George Dunning, who animated and directed Yellow Submarine, and they'd got their hands on the last scene, the triptychs. They just showed that part, without music and with the projectors misaligned. Even so, it was astonishing.

Dunning told me: "We've sent it back, but we did copy it. And I think you ought to look after it." So began the restoration proper on 35mm. I was known at the Cinématheque Française as le voleur (the thief). I had prints of it from all over the world, all piled up, and every one had something to add to my restoration.

Mercifully, and you won't believe this, they thought it was going to be a smash hit back when it was first made, so there was a printed script, albeit in French. That was a wonderful guide, although there's a lot in it that wasn't shot, according to Gance. At least I now had the right order.

At Telluride film festival, we showed our restoration and Gance watched it from his hotel room window. I don't know what kind of view he had, but he stood throughout the whole thing – a man in his 90th year. There was a misunderstanding at one point: he thought a documentary was being presented as the restoration and he got very angry. Oh that was awful. But everything was fine once he had seen the whole thing, even if it did take till three in the morning.

I'm proud to be bringing people back to the cinema, in an age when people will happily watch Lawrence of Arabia on their mobile phones. Napoleon is pure cinema, and cinema was designed for sharing. There's something about the way it was shot that makes it like no other. I can't tell you how many people, having seen our restoration, have said: "That was the greatest experience I have ever had in a motion picture theatre."

Carl Davis, composer and conductor of the restoration's score

Conducting a silent film requires a lot of concentration. You lose track of time. The score I wrote for Napoleon is very long, but I'm getting better at conducting it. When it was first proposed, I had no one to turn to and ask: "How do you create a score that's going to run for five hours?" I had to reinvent the process for myself.

I'd been working with Kevin on a Thames Television series called Hollywood, all about silent cinema. It gave me the opportunity to meet people from the silent era. The most interesting was Ann Leaf, a lovely little woman who lived in a house just behind the Hollywood sign. She had a big cupboard full of music and would start pulling pieces out, saying: "This is very good for chase sequences, this is very scary music, this is for a love scene and this is for Roman orgies."

When we came to the end of making Hollywood, I said very loudly at a celebration party: "Now that I've written about 300 bits of music for a TV series, why don't we screen an old silent and I'll write the music for it?" And Kevin suggested Napoleon, probably the longest film ever. So that was how it all started.

In the end, I had three and a half months to write the new score. The original, by Arthur Honegger, was lost. I thought it would be really interesting to make it like a counter performance, a portrait of the music of Napoleon's time, using the work of Beethoven, Mozart and Haydn. They wrote very dramatic music, but wigs-and-corsets stuff can only do so much. So I would tell my assistant: "I want you to orchestrate this variant of Beethoven, but can you pretend that you are Schoenberg doing it?" We amplified, perverted and did all sorts of things to the classical pieces.

Although Napoleon's big "cineramic" effect is made by three projectors, I was working off a normal 1980 TV. So at that first performance, I wasn't prepared for when the thing becomes three times larger. I remember doing some big chord and thinking: "I don't have enough here." At subsequent performances, I brought in a giant organ and made as big a noise as I could muster.

I'm still in terror, you know – afraid that, as archives open and people find things in attics, the film will keep getting longer and I will keep having to add more music. But as far as we know, we have tracked down all but three or four seconds.

There's nothing that matches the experience of going along to see it. It's incredible. Word has gotten round: this is fun, this is extraordinary.