New York was where they wanted it to be. Unfortunately his arrest in America for cannabis possession six months before meant that John Lennon and his new bride, Yoko Ono, couldn't get the visas.

So they tried the Bahamas. It was sweltering. Spending a week in bed there, they soon decided, would not be fun.

That was how Give Peace A Chance ended up being recorded in Montreal - which is, after all, barely 400 miles north of Manhattan. On May 26, 1969, John and Yoko checked into the Queen Elizabeth Hotel to stage their second bed-in - a seven-day press “conference-cum-happening”, during which they would preach world peace to the world's press, while remaining horizontal on their king-sized divan.

Gail Renard, 16, was astonished when she heard. “Nothing ever happened in Montreal. I can insult it now by saying probably nothing has happened since. We were at school, and we suddenly heard John Lennon was coming.” She and a friend headed to the hotel. Already, it was surrounded by fans. “I would have given up, but my friend said, ‘Come on, we'll get in'.” They climbed the fire escape to a floor above Lennon's suite, clambered inside, then went back downstairs to the right floor, where they waited for their chance: “It was a Mission Impossible routine.”

When the security guards changed shift, Gail says: “We rushed and knocked on the door and Yoko answered. She was with her daughter, Kyoko, who must have been 5. I remember Yoko saying, ‘Come in' - and finding myself in a room with John Lennon. It was overwhelming. They had been travelling and he was very hungry, and for some reason they couldn't get room service. I had chocolate in my handbag, and I said, ‘Do you want the chocolate bar?' It sounds daft, but he was touched, and said, ‘Really?' We got to chatting.”

Instead of turfing her out, Lennon asked Gail if she wanted to conduct a radio interview with him. The problem, he said, was that it wouldn't be until that evening: could she stick around? “I had to ring my mother. She ended up speaking to John. You wouldn't want to cross my mother - a tough little lady.” Permission granted, Gail joined the entourage and ended up staying for the week-long bed-in, going home only at night.

By day, the suite was packed with press crews interviewing Lennon, and Gail sat in. When Derek Taylor, the publicist for the Beatles' record company Apple, shooed the journalists out, visiting acolytes including Timothy Leary, Petula Clark and Allen Ginsberg would stay on to party. “As the evening wore on John would say things like, ‘Right, you look tired. We've an early start tomorrow, so you should get to bed'. Probably all the good stuff was happening after I was away. My mother, I think, had said, ‘Make sure there are no drugs around her', there was none of that. One of the press made a pass at me, once - I was very naive, and he kissed me - but John intervened.”

Sometimes, Gail would take Kyoko out to play in the park. She remembers being impressed at how impossibly exotic it was that everyone drank wine at lunchtime. And she was not a little starstruck, not only by John, but by the American musician and TV host Tommy Smothers - one half of the Smothers Brothers - who played on Give Peace a Chance. Ginsberg and Leary she could take or leave. All the while, Gail was snapping away with her box Brownie camera. She has more than 150 photographs.

Lennon announced that he wanted to record a song in the room. “I got busy. He wanted some tambourines. With a bit of lateral thinking I rang the Salvation Army, but they weren't forthcoming. The Hare Krishnas were next. I phoned their headquarters and they came over with tambourines - and vocals.” It was on the afternoon of May 31 that John wrote Give Peace a Chance. “He had written it in his head already, because he just wrote the words down straight. I can see him now. He sat on the floor. When he was done he said, ‘I want it bigger so that everyone can see it'.” Lennon gave Gail the lyrics, which he said she could keep, and asked her to write out an idiot board. “That's the version that is in the film.” The recording was made on June 1, with the Krishnas on tambourine, Smothers and Lennon on acoustic guitar, and Leary and the rest cross-legged at the foot of the bed belting out the chorus. The day after that, it was all over. “John gave me a card, a magic card, with a phone number. He said, ‘If you ever need anything, phone this number,' and wherever he was in the world they would get him.”

Gail, a TV and radio writer who won a Bafta in 2001, kept the lyrics on the wall of her London study for years. Now she has decided to sell, and Christie's will soon announce that it is to auction them, along with some of Gail's original prints.

Looking at one photo, printed here, she says: “I just got into bed with them. How comfortable I look! Can you imagine if a pop star mentored a 16-year-old now? But it was all love and peace and people believed it. Thanks to John and all that I became braver. It made me think you can change the world, or at least your bit of it, and you should always try to.”

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