Last year, Sandra Kerr retired from her job at Newcastle University, where she had taught a course on “traditions of these islands” for the music department. “Through the whole 17 years,” she says, “I’ve had students say to me that Bagpuss was their first introduction to traditional music. That’s enormously pleasing, to think that young people who have gone on to get attention and admiration for their music might have started with hearing the music for Bagpuss.”

Bagpuss, of course, was the most important, the most beautiful, the most magical saggy old cloth cat in the whole wide world. He lived in a shop window, deposited there – along with the mice and their mouse organ, the carved woodpecker bookend known as Professor Yaffle, Madeleine the rag doll, and Gabriel the toad – by a little girl called Emily, who brought back objects that would inspire flights of fancy as the toys came to life. Oliver Postgate and Peter Firmin’s stop-frame animation ran for only 13 episodes in early 1974, but through repeats imprinted itself on the consciousness of successive generations. And alongside the magical stories, the music made for the series by Kerr and her then husband, John Faulkner, seeped into the imaginations of musicians and music lovers. Now all the music they recorded for Bagpuss – alongside outtakes and alternate versions – is getting the reissue it deserves, in a beautiful book-style package, from Earth Records.

The mouse organ. Photograph: Chris Radburn/Rex Features

‘One of the most recognisable pieces – the mouse round, We Will Fix It – dated back centuries.’ Peter Firmin making the Mouse Mill.

Kerr and Faulkner were, on the face of it, unlikely candidates to soundtrack a children’s TV series. They met in 1963, through the folk scene – both were at first enraptured by skiffle and US roots music – but something else happened that changed their musical orientation. “I went off to live with Ewan MacColl and Peggy Seeger, to be their au pair and folk apprentice,” Kerr says. “That does sound like an extraordinary job description, doesn’t it? I was completely and utterly over the moon with that. I had one-to-ones with Ewan in things like the social and political history of folk songs. With Peggy, I had formal musicology and guitar lessons. I was very fortunate indeed to have that year; it was my folk degree.”

The young couple were invited by MacColl into an informal gathering he established, the London Critics Group. “We felt we had a mission to promote British folk songs and folklore, which were being ignored because of the influence of American music,” Faulkner says.

“The first thing I always think about was how exciting it all was,” Kerr says. “There we were with one of the architects of the folk revival, and with Peggy, and every week we were in their house listening to traditional singers, analysing what they did, singing ourselves, and learning to critique what they did. We studied hard and worked incredibly hard.”

‘Because of what we were presenting, he started to think of it as music-led.’ John Faulkner and Sandra Kerr. Photograph: Smallfilms

The pair have different memories of how, in 1973, they came to be approached by Postgate. “Oliver, who didn’t know me and Sandra, knew Ewan.” Faulkner says. “I think there was a connection through his father, Raymond, who was a socialist historian. I think Oliver contacted Ewan and explained he was doing a series, and asked if Ewan could recommend anyone who might be interested in making music for it.”

Kerr’s version begins with her and Faulkner supplying the theme for Sam on Boffs’ Island, a series written by Michael Rosen for schools television, starring Tony Robinson, Miriam Margolyes and Michael Rosen. “The producer, Claire Chovil, had lunch with Oliver. He said he was doing a new series about a saggy old cloth cat and he wanted a different kind of music for it. She said she had just worked with a couple who had played hundreds of instruments, which was us.”

Perhaps, though, Postgate knew who they were anyway – for he and Firmin had supplied animated sequences to Sam on Boffs’ Island. But what Kerr and Faulkner agree on is that neither of them had met Postgate before they travelled down from London to his home in Kent to discuss Bagpuss. “Red Lion House used to be a pub – which was a very good sign, especially to folk singers – and we got on like a house on fire,” Kerr says. “John and I were very leftwing: Oliver’s father had written the definitive history of the trade union movement, and his uncle was [1930s Labour leader] George Lansbury. We had all kinds of links.”

Faulkner says: “He explained what these programmes were about, and they were sort of experimental, in the sense that he liked the idea of using music that was traditional-based, because the stories he had written were very folky. He wanted something more bedrock, and we could provide the kinds of instruments I don’t think he’d considered – autoharp, mandolin, concertina.”

Both remember making the Bagpuss music to be a delight. Postgate would tell them the stories he was telling in the episodes, sometimes supplying them with lyrics to set to music, and they would compose around his suggestions. “He would give us a lyric, like The Bony King of Nowhere,” Faulkner says, “and say: ‘Can you write a melody for that?’ We would go away and throw it around, then record a tune to cassette. Because of what we were presenting, he started to think of it as music-led. I think it’s true to say that he was inspired by what we were giving him, and we were certainly inspired by what he was giving us.”

Curator Alice Sage works on the Smallfilms retrospective at the V&A Museum of Childhood, 2016. Photograph: Jonathan Hordle/Rex/Shutterstock

Photograph: Smallfilms

The pair were inspired by many different strands of traditional music of the British Isles: the Irish traditional song Brian O’Lynn makes an appearance; for The Weaving Song, they set Postgate’s words to an old Scottish tune called The Calton Weaver. One of the most recognisable pieces – the mouse round, We Will Fix It – dated back centuries. “I think we had just recorded Sumer Is Icumen In with the Critics Group,” Kerr says, “and, when Oliver said he wanted a little round, that must have been in my head.”

The results were magical. The unusual instrumentation – with concertina, Appalachian dulcimer and autoharp prominent – gives the music a timeless, otherworldly feel that is almost hallucinatory in its quality (in his sleevenotes for the reissue, Stewart Lee suggests that it has influenced acid folk). And the very best of the music – The Miller’s Song, for example – is so good you can’t believe it was recorded for a lunchtime children’s TV programme. “That’s my favourite,” Kerr says. “And Oliver loved that song in particular. I still sing it, and people respond to it whether or not they were Bagpuss fans, so I suppose it has entered the folk tradition, which is very gratifying.”

Of course, the original audience for Bagpuss barely comprehended the music in their infancy, but as they grew older, its wondrousness became apparent. “It wouldn’t surprise some people to hear that in the days before iPads, when we went on tour we used to watch videos on the bus, and one of the regular things was Bagpuss videos,” says Sarah Martin of Belle and Sebastian. “I remember sitting on the bus watching it with Chris Geddes and The Miller’s Song came on, and we both said: ‘What a great song.’ You’re not aware of it as a kid, because it’s quite wordy and meaningful, but to find a gem like that on a children’s TV show is extraordinary.”

• The Music from Bagpuss is out now on Earth Recordings.