After months of work on his score for Undergound (1928), Neil Brand is still, happily, a big fan of the film. In fact he’s enthusiastic, and generous, enough to offer Silent London a preview of the music ahead of the world premiere next Wednesday and to chat about the film, and the process of scoring it too. Anthony Asquith’s film is set in London, but borrows its visual style from the European and Soviet art cinema that he loved so much: expect dark shadows, quickfire editing and geometric compositions. “Asquith was never again so bold as he was with Underground,” Brand says, and this score represents Brand’s attempt “to make music as bold as the film is”.

It hasn’t been an easy task. At first, he says, he was intimidated by the task ahead: the difficulty about writing for Underground, as opposed to Blackmail, which Brand scored for the BBC Symphony Orchestra last year, was that Asquith’s film requires snatches of lighter music. Blackmail is like an “icicle to the heart”, but Underground has wry, comic moments, at least towards the beginning of the film, before the characters make some disastrous decisions, and the film’s romantic triangle becomes an “Expressionist nightmare”. “Those first 20 minutes were horrendous to write,” he says. But four months later he has a complete score, which will be played by the BBC Symphony Orchestra, conducted by Timothy Brock, at the Barbican Concert Hall next month.

Brand is of course known for his piano scores, often improvised, for silent films, and here he has incorporated a piano into an orchestra score for the first time. He tells me this is partly because he wanted to use the love theme he had written for the film when he accompanied it at the London Film Festival with the Prima Vista Social Club two years ago. He also wanted to use the piano’s percussive bass sound and he enjoys the sound of a solo piano, at moments, over a quiet orchestra. “It’s almost a Morricone effect.”

Other than that though, Brand tackled the score as he always does, from the beginning to the end. This means that every morning, before starting work on the next segment of the film he would play through the existing score from the start. So he has heard the opening of the score, on his home computer setup, many, many times.

And what an opening it is. I was listening to a computer simulation of the orchestral score, with the film running in a tiny 3in window in the corner of the screen. Luckily, the music speaks volumes, with sweeping harps and chiming bells that immediately tell us we’re in London town. These first minutes play like a short overture, running through the changing moods of the whole film. The love theme makes an early appearance as the title of the film is revealed, but sharply gives way to chase music as the camera plunges down to the subterranean tunnel and races into the tube station. Before we know it we have come to a comic scene, as commuters jostle for seats, and flirt a little too. This is a moment when the film pauses to enjoy a touch of comedy, and the score offers a hint of a hornpipe as a sailor saunters into the carriage, for example.

But the tone isn’t always so light-hearted. Brand uses the full force of the string section to create shimmering, celebrational music as characters … use an escalator. But it’s more than that, of course. Asquith shoots the escalator and the crowd, from a low angle, creating grandeur, giving Brand the opportunity to match the feeling of awe with his music.

The score can be character-led too. When we first meet Kate, the spikiest corner of Underground’s love triangle, low notes on a cor anglais strike a sinister tone. “All music has to have an opinion,” says Brand, and the score does sometimes tell the audience what to think. Here, we know that Kate is set on a tragic course – and that she is a troubled young woman. Later, as her actions come back to haunt her, she is accompanied by harp, deep piano and vibraphone playing snatches of themes from earlier in the film, with random pizzicato to represent her distracted, anguished, state of mind.

For an example of how Brand’s score drives Asquith’s visual ideas home, you could do far worse than enjoy this snippet of the film’s rooftop chase sequence. Bear in mind, of course, that the music you are listening to here is from a computer, not a full-blown orchestra:

That’s Underground: humour, romance, action, melodrama and fear. “Asquith isn’t interested in a homogenous feel,” says Brand. He feels that his task when writing the score was to emphasise each of these changing moods fully, in order to do justice to the film, and that means that the score, too, has to turn on a sixpence.

Brand shows me the edits in one bar-room fight sequence to illustrate the point: eight shots in 12 seconds, including a fist coming right at the camera. “It’s a lovely challenge for me,” he laughs. Each new shot is marked on Brock’s copy of the score, alongside the usual musical notation – and there are at least 70 non-musical cues such as bus bells, door knocks and a cymbal-hit as one character flicks another’s collar. Brock, and his 85-piece orchestra, have to meet all of them on the dot, or the picture and the score will seem to be out of synch.

There is no “synch”, though. It’s all an illusion, created by Brand’s score, Brock’s conducting and a hugely talented – and huge – orchestra. Underground promises to be spectacular.

Underground screens with the BBC Symphony Orchestra at the Barbican Concert Hall on 5 October at 8pm. The film will be followed by a panel discussion, chaired by Francine Stock and featuring Neil Brand, Matthew Sweet and Robin Baker from the BFI archive. You can buy tickets here, on the Barbican website.