The Beatles' 'Sgt Pepper's Lonely Hearts Club Band' LP is 20 years old this month, a milestone for musicians, studio techniques, and 4-track technology. It was arguably the first concept album, and producers, engineers and players can still learn lessons from it today. That's why John Morrish and Tony Bacon, dressed up in their best velvet uniforms, tell the story of just one track — the LP's finest, 'A Day In The Life' — from fresh interviews with producer George Martin and engineer Geoff Emerick.

A Day In The Life is the last great Beatles recording, although no-one would have expected that at the time.

It's one of the few actual collaborations between Lennon & McCartney, with George Martin an essential third partner. Without the recording he directed, the song would be nothing more than guitar strums and rambling words.

It's also a much better song than its creators intended. That sounds odd, but it's obvious. We say things all the time that reveal more than we really intend. And our words may mean more to other people than we can anticipate. So it is with songs.

Lennon's verse is a dirge in E minor, the opening "I read the news today, oh boy" a snippet of blues scale. Lennon tells us that he sees a rather sad news item about a man getting killed in a car crash and he just has to laugh.

Clearly, it's an inappropriate reaction, the kind of thing you see in heavy drug enthusiasts, the psychologically disturbed and the heroes of existential fiction. Lennon, at various points, was all three.

The music and Martin's production reinforce the mood. It's difficult to imagine the three central figures sitting down and discussing what the song was supposed to come out like: it happened anyway.

For the second verse, the drums arrive, sounding like orchestral tympani but played like a normal kit. They drive a compressor which, in turn, winds the acoustic guitar level up and down so that it is squeezed out between drum beats, like toothpaste from a tube.

The verses end with that famous banned line "I'd love to turn you on", rocking between B and C notes, a siren wail akin to the one in I Am The Walrus, and no less sinister. It sits unhappily over the Bm, G and Em chord sequence.

The "boys" had already put together Lennon's verse and McCartney's middle section, but between the fragments they wanted "something else". Paul marked out a 24 bar stretch on the 4-track by banging out a piano E four-to-the-bar throughout. They told George to book an orchestra and make them freak out. He declined, writing out 24 bars of upward slide instead and telling the half-sized session orchestra to play without listening to each other.

The material went down on a separate Studer 4-track to that used for the band's playing. Four, or perhaps only three, tracks of orchestra were recorded, in time to McCartney's piano, using for the first time a link between the two machines, invented on the spot by EMI engineer Ken Townsend.

Opinions differ as to how effective it was. The machines would run in sync, the second being driven by a 50 Hz tone recorded on the first, but they wouldn't start together. The orchestral tracks were recorded out of time with the original song, and with each other. And the same problem cropped up at the mix.

Consequently, the rhythmic pulse of the music falls apart during the orchestral passage. No doubt infuriating at the time, it looks now like a happy accident, as disorientating as Lennon's lyric.

Then there's a blunt edit and an alarm clock goes off. A joke by roadie Mal Evans intended to point out the end of the 24 bar gap, it is preserved for posterity because it got onto the tape via the foldback speakers. It ends up as part of the song, with McCartney singing "Woke up..." immediately afterwards.

Paul takes us on a trip on a bus. Upstairs he has a smoke (a cheer from the rest of the band is clearly audible) and goes into his dream. Actually, it's George Martin's dream, brass, strings, echoed "Ah"s, all making the kind of psychedelic music that was as much to blame for the BBC ban as Paul's lyrics. Not bad for a man who much preferred a cup of tea.

The session men hated the Beatles' experiments. Silly people: this was probably the most important recording any of them ever made. George's powers of persuasion were stretched to the limit.

After the last verse, Mal Evans returns to count bars again, but he's soon drowned by the orchestra. His voice this time is accidentally committed to tape, becoming a "found object" before Eno ever got to art school.

The strings scream like a B52 on the runway, arriving at a chord that is approximately E major, being made up of the top notes of each instrument. A moment's pause, then the final crashing chord, a roomful of pianos playing E major in a voicing known only to Uncle George. In classical music, E major sometimes signifies heaven. The Beatles only knew, probably, that it felt right. When you've been playing a gloomy E minor, E major has got to be a joyful release.

Up come Geoff Emerick's faders, holding the chord for a full 42 seconds and making the Abbey Road air conditioning part of the production, just like the words, the music, the weird instruments, the performances, the accidents and masses of unheard engineering expertise.

The pop musicians of 1967 weren't artists: they certainly didn't think they were. The tools they needed hadn't been invented. They didn't really know what they were doing, just that they had to do it. Today, tape machines stay in perfect synchronisation. Alarm clocks don't go off in studios. Accidents have to be arranged.

Today's records are made with calculation and with the end product always in mind. One person can do it all. It's a hefty responsibility to know that every sound on the record is going to have to be put there, deliberately, by you.

It wasn't like that for Lennon, McCartney and Martin. They just got together in a room and made the record that is their highest achievement. They didn't mean to.