This 1974 program by Ann Bonsor & Humphrey Carpenter interviewed many of Tolkien's friends, and inspired Carpenter to write his Biography. Details from it have been quoted multiple times in various books on Tolkien, like Edith brought food for Tolkien brothers, Coghill heard Tolkien reading FoG in 1919, Treebeard's Hrum was from CSL, Elaine Griffiths's role in publication of The Hobbit. However, it appears few had the chance to listen to the original program, save for some excerpts used in Tolkien: An Audio Portrait. (Tolkienbook.net suggests it was also rebroadcasted in 1975 on BBC radio 3.)

Recently I came across a 1 hour NPR program in 1975 posted by Gordon Skene, a collector of 150,000 radio tapes, via the amazing blog/magazine Tolkien Gleanings. And I realize it is probably identical to the 1974/5 BBC one (perhaps Findegil could confirm it?). It interviewed Priscilla Tolkien, Neville Coghill, Gervase Mathew, Elaine Griffiths, Rayner Unwin, Joy Hill, Lord Halsbury, and Charlie Carr. I found many accounts in Biography and Companion and Guide are actually based on it, and it contains so many anecdotes that I have never heard of: Tolkien saying he is Bilbo the historian, Stanley Unwin wrote triple underline in his "If you think this is a work of genius, then you may lose a thousand pounds", Edith sent food to Tolkiens's room via a pulley system (therefore the boys didn't go to Edith’s room as Carpenter said), how CSL hrumed...

I made a rough transcription on Descript, enjoy!