Paul had always enjoyed a close relationship with John's son Julian, then five years old and, to show his support for mother and child during the break up, he drove down to Weybridge from his home in St John's Wood bearing a single red rose. Paul often used driving time to work out new songs and, on this day, with Julian's uncertain future on his mind, he started singing and improvising lyrics on the theme of comfort and reassurance. At some point during the hour-long journey, Hey Julian gave way to Hey Jules and Paul developed the lines "Hey Jules, don't make it bad, Take a sad song and make it better". It was only later, when he came to flesh out the lyric, that he changed Jules to Jude, feeling that Jude was a name which sounded stronger.

The song then became less specific. John always felt it was addressed to him and was Paul encouraging him to make the break from the Beatles and build a new future with Yoko. Paul felt that, if anything, it was addressed to himself and the adjustments he knew that he was going to have to make as old bonds were broken and new ones forged.

As with so many of Paul's songs, it was the music that drove the lyric, with sound taking precedence over sense. One line in particular — "the movement you need is on your shoulder" — was seen only as a temporary filler. When Paul played the song to John on July 26, 1968, he pointed out that this line needed replacing, saying he knew that it sounded as if he was singing about his parrot. "It's probably the best line in the song," said John. "Leave it in. I know what it means."

Julian Lennon grew up knowing the story behind Hey Jude but it wasn't until 1987 that he heard the facts of the composition first hand from Paul, whom he bumped into while staying at the same hotel in New York. "It was the first time in years that we'd sat down and talked to each other," says Julian. "He told me that he'd been thinking about my circumstances all those years ago, about what I was going through and what I would have to go through in the future. Paul and I used to hang out quite a bit — more than Dad and I did. Maybe Paul was into kids a bit more at the time. We had a great friendship going and there seem to be far more pictures of me and Paul playing together at that age than there are pictures of me and Dad."

It's only fairly recently that Julian has started asking questions about his past. Paul was right to anticipate that he would have a hard time growing up. "I've never really wanted to know the truth about how Dad was and how he was with me," Julian admits. "I didn't want to know the truth and so I kept my mouth shut. There was some very negative stuff talked about me — like when he said that I'd come out of a whisky bottle on a Saturday night. Stuff like that. That's tough to deal with. You think, where's the love in that? It was very psychologically damaging and for years that affected me. I used to think, how could he say that about his own bloody son!"

Julian hasn't studied the words of Hey Jude for some time but finds it hard to get away from the song. He'll be in a restaurant when he'll hear it played, or it'll come on the car radio when he's driving in Los Angeles. "It surprises me whenever I hear it," he says. "It's very strange to think that someone has written a song about you. It still touches me."

Hey Jude was the most successful Beatles' single ever. It topped the charts around the world and, before the end of 1967, over five million copies had been sold.