The group was naturally nervous about performing live for the first time in almost a year, let alone appearing before the largest audience ever assembled at the time. Leaving nothing to chance, it was decided that a backing track was necessary to ground the performance, thus making the TV spot a fascinating glimpse into an actual work-in-progress.
Work began on the backing track on June 14th, 1967 at Olympic Sound Studios. John banged out the basic chords of the song on a harpsichord, Paul played a double bass and George picked up a violin for the first time in his life. Amazingly, all of these parts were kept in the final mix of the recording. Overdubs of more conventional instrumentation and layers of backing vocals were added at Abbey Road Studios over a number of days before June 25th, the date of the broadcast.
It is worth the effort to seek out the Beatles' segment from the original Our World broadcast on YouTube. It is presented in glorious black and white and lasts just over six minutes in length, starting with the boys in mid-song adding more backing vocals. A camera in the control room then picks up producer George Martin and engineers Geoff Emerick and Richard Lush as Martin cuts the singing and orders the tape rewound and the orchestra brought into the studio. TV announcer Steve Race makes such silly comments as "...you'll notice the musicians are not rock and roll youngsters. The Beatles get on best with symphony men." Once all is set, the tape plays the backing track and the magic begins.
The pack of friends and rock luminaries sitting on the floor around the Beatles look rather bored during the early parts of the proceedings, but once the orchestra starts playing Martin's clever score and the singalong begins, the party atmosphere kicks in and the Summer of Love officially has its anthem.
Later that night, John rerecorded his lead vocal, Ringo added a drum roll to the introduction and a new mix was made so that the song could be rush-released as a single. With such unprecedented publicity, it came as no surprise that the record was an instant worldwide number one bestseller.