Paul, Elton John and Julian Lennon were interviewed by Sean Lennon for a BBC radio 2 programme broadcast over the weekend in honour of John's 80th birthday. It includes a clip of Paul singing a never before heard Lennon-McCartney composition called, "Just for Fun".
https://www.bbc.co.uk/programmes/m000n5pr/episodes/player
(May only be available in the UK)