Childhood friends McCartney and Lennon fell out when the Beatles split in 1970, but had mended fences before John was tragically shot and killed in December 1980.

And speaking to Sean for BBC Radio 2's John Lennon at 80 series, Paul smiled: "I always say to people, one of the great things for me was that after all The Beatles rubbish and all the arguing and the business, you know, business differences really ... that even after all of that, I'm so happy that I got it back together with your dad.

'It really, really would have been a heartache to me if we hadn't have reunited. It was so lovely too that we did and it really gives me sort of strength to know that.'

While Paul and John teamed up to write some of the Beatles' biggest hits, such as Love Me Do and All You Need is Love, their songwriting process wasn't as successful in the early days of the group.

"There were a few songs that weren't very good," Paul said. "There were a few that were clearly (by) young songwriters who don't quite know how to do it. There was one called Just Fun."

Paul also revealed that despite the fact John died 40 years ago, his legacy lives on as he still influences his songwriting process.

"I'm writing something and I go, 'Oh, god, this is b**ody awful.' And I think what would John say?" he said. "And you go, 'Yeah, you're right. It's b**ody awful. You've got to change it.' And so I'll change it, and I know from reports that he did similar things to that. If I'd have a record out, he'd go, 'B**ody hell… got to go in the studio. Got to try and do better than Paul.'"