"We weren't very popular, let's put it that way," says Yoko today, sitting in the corner of a sofa in a suite at Claridges Hotel. "But we weren't thinking of whether we were going to be popular or not. We just felt that it was important for us to stand up for some issues, because maybe we could make a difference." In October 1971, not long after that first post- nuptial declaration, Lennon's most enduring recording, "Imagine", was released. "For us," says Yoko, "it was like a summation of what we believed."
Twenty-eight years later, "Imagine" seems to have become a kind of secular hymn. In the early days, the World Church asked Lennon if they could use the song and change the lyrics from "Imagine no religion" to "Imagine one religion". John told them that they didn't understand it at all. Now the lyrics have been voted Britain's favourite in a BBC survey, and in the recent HMV Music of the Millennium poll - the largest of its kind ever conducted - Lennon himself was voted "best songwriter" and "most influential artist". In addition, "Imagine" has scored highly as one of the songs most requested by the public to be played inside the Millennium Dome. In as much as the Dome could be seen as a cathedral to popular taste, the sentiments of "Imagine" could now be regarded as a non-denominational anthem for world humanism.
Yoko thinks it perfectly understandable that the song should have gained this status. "I think that `Imagine' is not just a statement to encourage social change. I think that it was almost prophetic. When you take a line from the song, `Imagine there's no countries, it isn't hard to do', for instance - well, we're getting there. When the Berlin Wall came down, I remember thinking, `OK, it's happening John' - that's just how I felt about it. And so I like to think that the song was a very positive vision of the future."
Dressed in black jeans and Chanel trainers, with a grey jacket, Lennon's widow looks far younger than her 66 years. She was born into a wealthy and socially prominent Japanese family, to a Buddhist mother and Christian father. The family spent much time before the Second World War travelling between Japan and the US. Deeply traumatised by her experience of the war in Japan, Yoko spent the Fifties developing her artistic practice before moving to New York in the early Sixties. There she became an early member of the avant-garde art scene and among the first to stage concerts in a loft. It was at her 1966 one-woman show in London that she met Lennon.
Today, she has an air of anonymous affluence (she is rumoured to be worth nearly $500m). But behind the pale-blue lenses of her silver-framed glasses, her eyes retain the intensity which you can see in photographs of her as a performance artist back in the early Sixties - and never more so than when discussing the significance of her husband's most popular song.
Politically, "Imagine" was an act of liberation for Lennon. Its Utopian theme and lyrical simplicity expressed his growing disillusionment with the New Left in Britain, many of whose causes he and Yoko had supported. As the radical intellectuals of the New Left had been eager to court John and Yoko as the perfect propagandists (a role which the couple had seemed to enjoy, albeit temporarily) so "Imagine" could be seen to challenge that faction's hard-line, political notion of social revolution.
"`Imagine' was something which most definitely did not satisfy the New Left. Or the New Right come to that," remembers Yoko. "It was a genuine wish, a dream for the future. It had a naivety about it, even on the level of the very simple chords, and the lyrics which anyone can understand. Actually, other songwriters have said that it's `so simplistic', or things like that, and of course we can all be artistic snobs. But this was one song which John really wanted to communicate to the world, and so he dispensed with his artistic snobbery and really made it very sweet and simple - to get the message across."
Artistically, the simple list of instructions within the song's lyrics share the technique of Yoko's "instruction paintings", where the viewer is requested to "imagine" a conceptual function of each piece. Throughout the Sixties these instruction pieces had played a central part in Yoko's own artistic practice. They took the form of either purely aesthetic statements like the clear glass Painting to Let the Evening Light Go Through or as highly political performance pieces like the 1964 feminist work Cut in which audience members were instructed to come on stage and cut off pieces of Yoko's clothing. This combination of artistic simplicity and political demonstration became hugely influential on Lennon: to oppose intellectualism with the literalness of "War Is Over If you Want It".
How much of this inspired "Imagine"? "Well, the idea of making a work in your head is something which I made in many instruction pieces, and so `Imagine' is the word which was used a lot in my  book Grapefruit," says Yoko. "And as we were artists, living together, we inspired each other. But I would sooner leave that to individual critics. But I do think that John, through his particular background, was different to the London intellectuals. He had a wisdom which came from his struggle to survive. I came from a background which was totally the opposite, in many ways, but it was also a background where one needed to exert an incredible amount of energy to rebel - so that you could come out and breathe in a larger world, so to speak. And so we had both shared that experience."
The "literalness" of John and Yoko's artistic technique brought them back to a wider public. At the time of its release, "Imagine" was far more of a commercially acceptable pop record than any of Lennon's previous solo albums. In Britain, where it spent three weeks at Number One before remaining in the Top 30 for another 16 months, it seemed to be more of a high-street record than an underground record. It could sit quite comfortably beside the latest LPs by David Bowie or T Rex, and it returned John Lennon to the mass audience who had felt alienated by his avant-garde recordings with Yoko, his use of the word "fuck" on "Working Class Hero" or the couple's controversial "happenings".
Paul McCartney - whose determinedly whimsical "Uncle Albert" had been a huge US hit at the same time as "Imagine" - made a comment to the media that "he liked `Imagine' ... but there was too much political stuff on the other albums." And John responded with a letter to Melody Maker: "So you think `Imagine' ain't political? It's `Working Class Hero' with sugar on it for conservatives like yourself!" Ironically, it was this "sugar" which made "Imagine" such an enduring success.
"`Working Class Hero' was not taken well," says Yoko, "regardless of the fact that John was an ex-Beatle and so on. But `Imagine' fulfilled people's dreams in a way. I think that it was pretty courageous for the song to say `no religion, too,' especially at that time. Even now, that would be courageous, because religion is still a source of fanaticism, and violence, at this point.
"John and I are still partners, in many ways, because we believed in the same kind of things and we were working for them together. And the songs that he's written are still in people's minds, as well as his statements, his ideas and his spirit - they haven't been forgotten. More than that, they're growing. And maybe that's the reason why `Imagine' is a song for the Millennium - that there's a kind of silent conspiracy growing, if you like, between all of the people who believe that we can all make a difference."
In Britain, at least, the massive Oasis fan base has introduced a whole new generation to the significance of John Lennon. Anyone who has watched the video of Oasis playing at Manchester City football ground will have caught the moment when Liam prostrates himself on the stage before a monolithic back-projection of Lennon, and the group have been evangelical about the Beatles since their very first interview. For Yoko, the younger generations are an inspiration and indication that social and spiritual change are not only possible, but happening already.
"I think that young people have a great faith, but just now they're more concerned with anger. And that's an anger towards a bad generation before them, maybe, for being so dumb. We have to accept that our generation has made mistakes: we're still cynical, we're still busy making money, and we don't really care about ecology or the planet. So of course the younger generation will be angry at us. But as long as they're just involved in anger, they are only absorbed in their experience of us. If they can forget about us, then they can just go on and do what's right for them. And maybe we can join them."
As she is talking and sipping her tea, Yoko seems to grow more confident. She is softly spoken, but her Japanese American accent lends her speech an authority which is not linked to the usual issues of class. She admits to being amazed at just how edgy she still becomes when she gives an interview, and her initial few statements are delivered in a series of faltering clauses, punctuated by short nervous coughs.
"I am also very concerned as a mother," she continues, "that I could become a very overpowering parent if I tried to share my experience too much with my children. I don't want to do that with my son Sean [Yoko also has a daughter, Kyoko, from an earlier marriage]. I think that he should have a breathing space where he can find out things for himself. John and I felt the same about Sean's development, that he shouldn't go to school until he wanted to go to school and so on. But then he wanted to go right away, and so off he went! I didn't want to cram him with my ideas and I'm surprised that he knows so much about John and his music. Also, he is a very New Age man - very considerate to the position of women in society and so forth, which, again, I didn't force on him."
It is easy to dismiss John and Yoko's Utopian views as just the well- intentioned beliefs of a couple whose extraordinary fame and wealth made them utterly remote from the realities of daily living. On the other hand, it was that very position which so determined Lennon to denounce all forms of gurus, from Buddha to the Beatles, and return to the uncompromising attitudes of his youth. His career would be a balancing act between the desire to rebel and his suspicion of all forms of organised belief - "I just believe in me - Yoko and me," as he sang on "God".
For Yoko, as the administrator of Lennon's cultural legacy, there is a constant need to ensure that material is released or repackaged in a way which respects its historic significance and cultural context. Next year will see the release of a major documentary about the recording of Imagine the album, overseen by Yoko, entitled Gimme Some Truth. Lennon fans will recognise this title from one of the angriest lyrics on the record. "I'm sick to death of hearing things, from uptight, short-sighted, narrow-minded hypocrites - all I want is some truth, just gimme some truth..."
"Things happen in a mysterious way," says Yoko. "First of all, when we made the Imagine record, we simply felt that we'd got enough songs and that it was time to go into the studio. And so we had a studio built next to the kitchen in our house at Ascot, so we could just go in there without even putting our shoes on - it was that casual. And then we decided to have somebody filming us recording the album, - and so there were nearly 14 hours of film. Later, we thought `Who's going to want to watch us making an album, at snail's pace - it's the most boring thing in the world.' So we chucked it."
Then, about two years ago, I remembered that we had all this film, and I wondered what we could do with it - never thinking that "Imagine" was going to be so popular again. But when I started watching it, even I was interested - and I'd heard the songs a thousand times over. So I got a top documentary film-maker from Los Angeles, Andrew Salt, to edit it down, and I went to Abbey Road studios to do the sound. And there are so many people in the film, Phil Spector is there, and George Harrison, Klaus Vormann, Alan White. It's that period, of course. Then there was a big party to welcome us in New York, and that's on the film as well. And just about everybody came. You see Jack Nicholson, looking very young. And Andy Warhol was there, of course and Miles Davis, looking beautiful. At the time, I suppose we just thought it was normal, but looking at it now, it all seems very exciting."
While Yoko is describing the new film, her manner seems to offer a glimmer of the socialite side to her life - the Lennons' close relationship with Warhol, for instance. The lingering hostility towards Yoko, as typified by Camille Paglia's side-swipes at her on a recent South Bank Show, accusing her of obscurantism or lack of humour, seems totally unjustified. Yoko has made a point of never discussing her private life, but has always been open about her political views. With regard to her art, she is as literal as it comes: her work makes concrete the term "what you see is what you get" and the titles are always direct descriptions. That said, her conversation is peppered with a certain visionary mysticism; she believes, for instance, that through information technology we can replicate a god-like status of being both invisible and omnipresent. She can also envisage a time when over-population may be solved by emigration to other planets. Her reasoning evolves from her belief that humankind is gradually solving the four sufferings described by Buddha: illness, poverty, old age and death.
Ultimately, "Imagine" the track has stood the test of time and the fickleness of public opinion. From the somewhat school-piano-like soft-pedalling of its opening chords, to the gentle power of its closing sentiments, it is difficult to resist its sincerity. Of course, the song is idealistic, and literal minded - but that is the whole point; and Yoko Ono remains committed to its creed of simple, "instruction" thinking: "I really think that we will have world peace, to the point where `peace' won't be used as a word any more, because it will just be the normal situation," she says. "There are really only two industries in the world: the peace industry and the war industry. And if the peace industry becomes more viable than the war industry then we will not have war any more. In the war industry, the people are totally unified - they just want to kill. Whereas the people in the peace industry - we are so critical of one another. We just have to learn to be more forgiving, more caring and more understanding of each other. The old idea of love and peace really does work. And if we can come together in the peace industry," she concludes, "then peace will happen. It's just so logical. And so I'll bet on that."
`Imagine' is re-released on Parlophone records tomorrow
Captions: Opposite: Yoko Ono, now 66, is currently working on a documentary on the making of the Imagine album, which was recorded at the Lennons' Ascot home in 1971
Above: John and Yoko during their 1969 `bed-in' for world peace. The lyrics for `Imagine', released two years later, were, says Yoko, `a summation of what we believed'