One of the joys of this series is that we span the whole of Robert’s career so far. We also get to enjoy the rare pleasure of the man himself talking about Led Zeppelin. It’s always an honour and an enlightening experience. This episode tackles a song from their legendary fourth album. A beautiful song, a wonderful lyric and a haunting double vocal from a folk music heroine (we also find out what Robert calls the album — IV? Runes? Untitled? Four Symbols? Or what?).