Inspired by the legend of Faust, the Gothic is the world's longest symphony, needing 1,000 singers and musicians – plus one scarecrow. Stephen Moss celebrates its resurrection at the Proms

John Grimshaw sets crossword puzzles for the Times. His other passion is the composer Havergal Brian. He thinks the two may be connected. "I've often felt there is something like puzzle-solving in Brian's works," says Grimshaw, who is chairman of the Havergal Brian Society. "You listen to a work 10 times, and then you realise one theme is derived from a theme two minutes earlier. It's like organic growth. It's not standard development. It takes a lot of teasing out."

Brian died in 1972 at the age of 96, and Grimshaw has spent four decades promoting the work of this great British composer, who wrote music for an incredible 80 years. He is about to see all his efforts come to fruition. This Sunday, Brian's Symphony No 1 – "the Gothic", the longest symphony in the world at an hour and three quarters, and also the most labour-intensive, demanding almost 200 orchestral players and a chorus of 800 – will be played at the Proms. This is only the sixth performance since it was completed in 1927, and Grimshaw believes that with the forces the BBC is gathering – two orchestras, 10 choirs and four top-notch soloists under the baton of conductor Martyn Brabbins – this could be the definitive interpretation.

Brian is a remarkable figure, little-known to the wider public but with a dedicated coterie of fans. Seats for the performance of the Gothic sold out in 12 hours, although 1,000 places remain for prommers. They should get there early: this is a once-in-a-generation opportunity, the first performance of the work in the UK since 1980.

Adrian Boult conducted the piece at the Royal Albert Hall in 1966. There is a famous photograph of him at rehearsals, issuing instructions through a megaphone, and when I meet Brabbins he tells me he has already ordered one for himself. Shouldn't he be using a clip-on microphone, à la Britney Spears? No, he says, the megaphone will be "a gesture", a nod to the great Boult.

The musical forces are so vast that the BBC had to book Alexandra Palace in north London for final rehearsals. "I knew it was a colossal undertaking," says Brabbins, "but I didn't realise quite how colossal." Proms director Roger Wright had originally wanted to use amateur musicians – three of the previous five complete performances have been amateur – but Brabbins quickly dissuaded him. "I felt that, with the incredible demands on the chorus, if the orchestra isn't totally secure, there might be room for slipping off course."

Sound the thunder machine!

The BBC National Orchestra of Wales and BBC Concert Orchestra were recruited, along with dozens of extra brass and percussion players. The Gothic not only uses every known modern instrument, its score also stipulates a "scarecrow", which makes a sound like a football rattle, and a "thunder machine", reflecting Brian's desire to go one step further than a metal thunder sheet. Brabbins has been rehearsing the musicians in Cardiff while the 10 choirs have been preparing separately around the country, ahead of a huge choral rehearsal in Birmingham town hall. Then it's on to Ally Pally, where all the performers gather for the first time, and eventually to the Albert Hall. It's the musical equivalent of planning the D-Day landings.

Brabbins, a delightfully unmaestro-like maestro, is unfazed by the challenge. "There are lots of Himalayan peaks and this is one of them," he says. "I can't envisage what it's going to feel like to move that big ship around. My hope is it will feel like any other concert once we've rehearsed. People think about conducting as this indefinable expressive art, which on the one hand it is, but I'm also an artisan. I've got the equipment, and I have to weave it into a perfect basket."

Brian wrote the Gothic in his 40s, in the wake of the first world war and the collapse of his first marriage. It comprises six movements. The first three, lasting almost 40 minutes and in effect a symphony in their own right, are purely orchestral. The other three are a setting of the Te Deum hymn of praise running to more than an hour. Brian also added a preface from Goethe's Faust: "He who ever strives with all his might/ That man we can redeem." The orchestral half traces the demonic striving and eventual fall of Faust, the choral his redemption, although it does end on a questioning note: "Shall I not be confounded in eternity?"

Brian said the work was inspired in part by his memory of singing as a boy chorister in Lichfield cathedral in 1887. According to Havergal Brian and His Music, by Reginald Nettel, the composer took as his subject no less than "the greatness of the universe and of man's place in it". Malcolm MacDonald, the leading authority on Brian's symphonies, called the Gothic "an evocation of a whole epoch in the human mind; a purely musical parallel to parts of Goethe's Faust; a compendium of musical history from medieval times to the early 20th century; a musical equivalent to the cruciform plan of a Gothic cathedral; a response at several different psychological levels to the experience of world war one, and much else besides."

It is tempting to link Brian's own striving with the Faustian figure at the centre of the Gothic. Born into a working-class family in Stoke – he kept traces of the accent all his life – he had a good practical musical education, studying violin and cello, and playing the organ in local churches. But he was self-taught as a composer and endured a lifelong struggle to win recognition. He also had a messy personal life, and in 1913 left his wife and their five children. The conventional account is that he was having an affair with the family's maid, though Nettel suggests Brian's wife, too, had taken a lover.

It's very hard on the knees

Brian eventually married the maid and had a further five children. When an early benefactor died, money became short, and Brian just about made a living as a music journalist and by copying out scores. All the time, he carried on writing, despite the fact his work was rarely performed. The Gothic, which took eight years to write, had to wait until 1961 for its premiere, and Brian himself did not hear it until Boult's 1966 performance. Asked at the end how he felt, he said sitting down for so long was extremely hard on the knees.

Everyone who knew him remarks on his indomitability, his absence of self-doubt. That was probably the only way he could have carried on composing against the odds. "He knew he was a great composer," says Lewis Foreman, the English music expert who knew Brian in the 1960s. "He had total self-belief." An autodidact who read voraciously and was so besotted with German culture that he taught himself the language, Brian seems to have lived in a self-created world that nothing could disturb.

The recording of Boult's performance includes an interview with Brian in which he is asked if he was ever discouraged by the indifference he suffered for 50 years after an initial burst of acclaim. "Not a bit," he replies. "The greatest interest I've had in my works was to get them written, not to get them performed."

"Do you have a philosophy of life?"

"I'm afraid I haven't," says the 90-year-old gently. "For 60 or 70 years, I've been trying to find out the purpose of life and I haven't found it yet."

Thanks to the advocacy of composer and BBC producer Robert Simpson, Brian had an extraordinary late flowering, writing 20 of his 32 symphonies after the age of 80 – another musical record. Unlike the Gothic, these are taut, concentrated works almost devoid of musical transitions.

Roger Wright does not claim the Gothic is a masterpiece, but he is eager for the present generation of music-lovers to experience it. He recalls hearing Ole Schmidt's performance, with the London Symphony Orchestra, at the Albert Hall in 1980. "It was a bit ramshackle," he says. "I want to hear a really good live performance and feel the impact it makes."

A great performance of Brian's musical colossus will show us just how cathedral-like the Gothic is – and whether Brian's lifetime of striving was truly redeemed.

• The Gothic is at the Royal Albert Hall, London SW7 (0845 401 5040), at 7pm on Sunday; live broadcast on Radio 3.