More than four decades after they burst to prominence in the 1974 Eurovision song contest, Abba are once again climbing the charts, while the film Mamma Mia! Here We Go Again is riding high having taken more than £9m in its first week at UK box offices.

No one is more surprised about the Swedish group’s success – about 400m records have been sold since that Eurovision moment – than Björn Ulvaeus, one of Abba’s two songwriting Bs.

“It is quite strange,” he says. “Intellectually, I can understand the huge figures of sold records. But emotionally, it is difficult to grasp that it has meant a lot to a lot of people. You and I are sitting here talking about it this – something I thought would be gone in oblivion two years after we split up. I still don’t know how it happened, but I’m grateful.”

After three-and-a-half decades apart, Abba are being reborn later this year, releasing two new songs as “Abbatars” – AI reincarnations of their late-1970s selves, with Agnetha Fältskog and Anni-Frid (Frida) Lyngstad performing the two compositions by Benny Andersson and Ulvaeus. The songs are “one pop song like you could have heard in the 70s and one reflective one”, Ulvaeus says.

At 73, Ulvaeus is a grandfather, with small spectacles, trim beard and a professorially precise manner. He once said he would never reunite the group, which split after his divorce from Fältskog and Lyngstad’s from Andersson, preferring to remember it as “young, exuberant and full of life”.

Facebook Twitter Pinterest Benny Andersson, Anni-Frid Lyngstad, Agnetha Fältskog and Björn Ulvaeus after making it to the Eurovision Song Contest final in 1974. Photograph: Olle Lindeborg/AFP/Getty Images

The AI idea, he says, helped the group defeat reservations about a comeback: “It’s a challenge and something no one expected. We’re creating heads of ourselves from 1979. The ladies chose that year. I gather they thought we all looked particularly good then.”

For a man who charted his own emotional ups and downs in songs, he’s keeping the habit going. “‘I still have faith in you’ is almost existential – and it reflects on our life now,” he says.

It took 10 years since the first Mamma Mia! film for Ulvaeus and Andersson to agree a second script, written by Richard Curtis and Oli Parker. “We looked at a lot of scripts. Finally with this one we thought it would be good to see this movie.” Do all four members of Abba have to agree? “This was about music and lyrics, so it’s Benny and me – but when it concerns Abba, we all four talk about it.”

In the Mamma Mia! Here We Go Again, an expanded cast of actors have another go, with varying proficiency, at Abba songs.

Lily James “sings like a goddess” and Amanda Seyfried “sings better than she did 10 years ago”, says Ulvaeus. “Christine Baranski has a great voice. There are also actors who don’t need to sing so much. Mostly, [the cast] sing very well.” In pause for honesty, he adds: “Mostly.”

Some lyrics may strike a less comfortable note in the era of #MeToo. Would he write Does Your Mother Know? (about a teasing encounter between a young discogoer who is “only a child” and an older man) today? “Maybe I would hesitate today. There was this young girl I must have met in my career, when I was on tour. But I never went across the limit. As you see in the song I told her stay off, take it easy you’re too young… My conscience is clear.”

Facebook Twitter Pinterest Björn Ulvaeus, Agnetha Fältskog, Anni-Frid Lyngstad and Benny Andersson in their mid-70s pomp. Photograph: Allstar Picture Library

For all the slinky white catsuits and platform boots, he says would not style the band differently. “We had fun and we were equals. But I think the #MeToo revolution in Sweden has changed so much – we are are thinking differently now. I’m a feminist. Always been [one]. Even when Agnetha and I and Benny and Frida got divorced, we did it on equal terms.”

It is Brexit that animates Ulvaeus most. “I’m a European through and through and really, really sad that some Brits don’t want to be with their friends. I just don’t understand it.” Abba was “such a European idea. We took our inspiration from French chansons, German Schlager, Italian ballads and Nordic folk.”

Ulvaeus describes his politics as “freethinker and humanist, because if there is some thing up there running there, we don’t know what it is, so we might as well get on with running things as humans, we have to take responsibility”.

Now in his eighth decade, he says: “I have more freedom, I am braver, I can make quick decisions and am not as worried as that young man was. I lift my eyes and see the bigger picture more then he did. If I give him advice it would be ‘don’t be so worried. Things will sort themselves out.’”

Asked to choose one piece of music to accompany him on a desert island, Ulvaeus hesitates. Abba wouldn’t make his final cut. “I would take a symphony by Beethoven,” he says.