Sir Paul McCartney plays the unheard first song he wrote with John Lennon, 'Just Fun', in an upcoming interview to mark the late Beatles legend's 80th birthday.

To celebrate the occasion, Sean ono Lennon - Lennon's son with Yoko Ono - interviews his dad's bandmate, McCartney, his godfather, Sir Elton John, and his half sibling, Julian Lennon, on BBC Radio 2 for the two-part documentary, 'John Lennon At 80', which airs on October 3 and October 4 between 9 and 10pm.

And while speaking to Sean, 44, about writing with his father, the 'Let It Be' hitmaker plays a snippet of the track on the guitar.

'Just Fun' is believed to have been written in 1957, and opens with the lyric: ''They say that our love is just fun / the day that our friendship begun.''

The song was mentioned in the 1970 film 'Let It Be' and was written when the pair skipped school.

McCartney recalled: ''There were a few songs that weren't very good ... you know, clearly young songwriters who don't know how to do it.

''Eventually, we started to write slightly better songs and then enjoyed the process of learning together so much that it really took off.''

McCartney, 78, also spoke of first meeting ''teddy boy'' Lennon - who was shot dead outside his Manhattan apartment in December 1980 - in Liverpool, and how the pair ''complemented each other''.

He said: ''I look back on it now like a fan, how lucky was I to meet this strange teddy boy off the bus, who played music like I did and we get together and boy, we complemented each other!''

Elsewhere, Sean and Julian - whose mother is Lennon's first wife, Cynthia Lennon - shared how their father got them into music from an early age, how the music legend's final album, 'Double Fantasy', is ''overwhelming'' to listen to, and the day they found out their dad was a Beatle.

Helen Thomas, Head of Station at Radio 2, commented: ''John Lennon is one of the Radio 2 audience's most popular and best loved musicians, so we're thrilled and honoured that Sean's first ever radio programme in which he talks at length about his father, alongside his brother Julian, Paul McCartney and Elton John, will be broadcast on our network.''

'John Lennon at 80' will be broadcast on Radio 2 and BBC Sounds, and will be available to listen again on BBC Sounds for 30 days.

Lennon would have turned 80 on October 9.