Performed by members of The Shout, members of Voicelab, The London Bulgarian Choir, Maspindzeli, The London Gay Men’s Chorus, Velvet Fist, Raise The Roof, British Gospel Arts, ten megaphonistas

9.15 Dusk. A speedboat carrying Carol Grimes, singing a mad Doppler chant, sets out from Westminster Bridge at maximum speed and hurtles down river. As it passes through Tower Bridge

9.20 Ten singers on Tower Bridge, armed with megaphones, (the ‘megaphonistas’) sing a kind of muezzin call, in response to which

9.23 A fire brigade boat sets out from Tower Pier, firing two jets of water into the air, followed at a short distance by a huge pontoon, pushed by one tug, pulled by another. The boats travel slowly up the river.

The pontoon carries six redoubtable London choirs and a Shout singer Melanie Pappenheim. As it travels upstream, the choirs sing, individually, folk songs from their repertoires.

The singing is amplified, speakers pointing to the south bank of the river, where a gradually accumulating audience gallops along to keep up with the boats. People quickly down their pints at riverside pubs and join the throng.

There is a rumour that a Sex Pistols tribute band is going to pass (the thirtieth anniversary of an impromptu gig they did on the river in 1977), but it fails to materialise, disappointingly.

9.30 The pontoon passes under London Bridge.

9.36 The pontoon passes under Southwark Bridge.

9.42 The boat slows down as it approaches Tate Modern.. From the pontoon Melanie sings a solo song Ich hatte einst ein schoenes vaterland.

9.45 The song ends. The speedboat comes hurtling back past upriver with Carol dopplering wildly. Manickam Yogeswaran (Yoga), standing on the Millennium pedestrian bridge, sings in response.

The choral singing on the pontoon resumes.

10.00 The pontoon passes under Waterloo Bridge.

10.05 The pontoon arrives in front of the Royal Festival Hall, and hovers.

The choirs on the pontoon (the ‘offshore chorus’) sing together for the first time – a stonking Georgian song, Mravalzhamier.

The fire brigade boat also hovers, sending out exuberant jets of water, then continues under Hungerford Bridge and disappears from sight.

10.08 A short and mysterious silence. Then the onshore chorus – 150 members of the wonderful Voicelab choir, many of whom sang with us at the Proms - sings from the level 5 balcony of the Festival Hall. They are invisible behind the parapet. It is as if the building is singing.

As the onshore chorus continues to sing, Jeremy Birchall, visible on the level 5 balcony, sings a solo.

The megaphonistas, ranged around the sides of the river – 3 on Hungerford Bridge, 2 on the north bank, 3 on Waterloo Bridge, 2 on the roof of Queen Elizabeth Hall, respond in turn, like a series of echoes from distant lands.

And again, Jeremy, with a more complex call, triggers the megaphonistas in turn.

And, as the sound comes back towards Jeremy, the onshore chorus pops up on the balcony and continues the response.

10.12 This leads to a call and response between the onshore and offshore choruses, involving Jeremy, Melanie and the megaphonistas. It’s a bubbling cauldron of choral sound.

10.14 Melanie sings from the pontoon, solo, I come across the footprints of seagulls.

A marvellous moment of stillness.

10.16 The onshore chorus responds with a celebratory song Vogel. They are joined by Carol and Yoga, who sing improvised solos. The pontoon moors up at the Festival Pier, and the singers of the offshore chorus disembark.

10.23 The song ends with a roar of confidence. The boats join in with their horns.

The audience moves round to the back of the building.

10.40 The choruses gather on the back balcony of the building, and sing a new song, gentle, solemn, beautiful - Der Mond ist Aufgegangen. After four verses, the singers take up the first verse again and walk into the building.

The singers continue singing for several minutes as they walk through the building, splitting into many small groups, taking the sound into the building and distributing it. A diaspora of sound. The audience follows.