Pink Floyd's groundbreaking concept album "Dark Side of the Moon" is a staggering 40 years old this month. As the BBC note, you could say its themes are just as relevant today - madness, money, identity, disorientation, all the stresses of the modern world. But in 1973, its construction was startling - with its musical episodes flowing one into the other, and its looped and layered sound effects. It's credited as one of the most influential rock albums of all time. Yesterday afternoon BBC World sat down with Nick Mason in his North London loft, and asked him whether they'd ever suspected the album would be so big...

You can hear the interview on demand through the following: