I was actually there when he wrote the song, but that's most definately another story for another time (hides from the hounding press)...
It's a little hazy to me now, but I think I remember the exact situation...
We were sitting in a room (God knows where!), and he looked up to the bunch of us who were hanging out with him (why no one brought a camera, I don't know... it just wasn't really that big a deal back then), just before his pen started moving... He said, "I want to write something... something... real... something lasting..." someone mentioned he write about the government, but others said nah, it was not him... then he said, "I'll write about liverpool. About my life" We sat in amazement as he wrote out the entire song in front of us... Then he picked up the guitar, fiddled around a bit, then suddenly, he got this look in his eyes... Then, bum bum, bumbumbumbum (the opening riff to the song)...
And a hit was born... It was really a great experience! And a beautiful song that Paul had nothing to do with whatsoever.