THERE'S a moment in A-Ha’s Summer Moved On (from their Minor Earth Major Sky album) when vocalist Morten Harket sustains a note for more than 20 seconds, said to be a record for a top 40 hit. It’s longer, even, than Bill Withers’ stunning effort on Lovely Day.

I mention this geek fact (20.2 seconds, for the über-geeks among you) because I fear to the casual pop fan, A-Ha are three good-looking guys from Norway who had a smash hit in the ’80s with Take On Me, made that video to go with it, and not much more.

Twenty-five years and nine albums later, you’ve got to give them credit for tenacity and staying power, if nothing else. This feat makes them the best-selling Norwegian music act in history and the second best-selling act from Scandinavia of all time, after the Nordic albatross that is Abba.

But they’re a world-class band by any yardstick and their unwavering belief in each others’ abilities is the bedrock of their success.

“When you’re a songwriter with someone like Morten you can go to places you couldn’t with other vocalists,” said guitarist Pal Waaktaar-Savoy, who has penned most of A-Ha’s hits, including Summer Moved On.

“He has an amazing ability and it’s always a pleasure. I remember when we recorded it that it was obviously a difficult thing to do. But I remember also the first time we did it live and Morten went on another five seconds!”

With 35 million album sales and 25 million singles shifted, Pal’s a heavyweight songwriter by anyone’s standards. His output includes A-Ha’s biggest hits, from the signature Take On Me, through The Sun Always Shines on TV to the band’s Bond theme, The Living Daylights and beyond. Way beyond.

You’d be forgiven for thinking they effortlessly emerged from the early ’80s synth band scene, alongside Depeche Mode, Human League and Heaven 17.

But even the global monster hit that is Take On Me took a while to get right. It took a third version and the groundbreaking, MTV-conquering, promo to get the planet interested.

“It took quite a while to get it to be a hit,” agrees Pal.

“The first UK release was a different version. We weren’t happy with the first one.”

That willingness to keep plugging away with the songs also bore fruit with later songs.

“The Sun Always Shines on TV was originally a ballad and Hunting High and Low was much more up-tempo,” says Pal, in a break from rehearsal for their farewell tour.

While it’s a given their music bears typically Norwegian traits – especially some of the lesser-known album tracks – keyboard player Magne “Mags” Furuholmen says there’s no way A-Ha would have been successful if they hadn’t rocked up in London when they did.

“The influence of the UK on our direction and music can’t be over-estimated,” says Mags.

“It really shaped us as a band. It was a very formative experience, coming to Britain. Without London A-Ha wouldn’t have happened.

“We were three Norwegians on tourist visas, we couldn’t work or employ anyone. We rented studios that had lots of synths because we had to do everything ourselves.

“What shaped the sound of A-Ha was the onslaught of interesting new music coming out of the radio in London, and also we needed to make records with just the three of us. There was no drummer, no bass player so we started programming.”

So why did they come?

“There was just one radio station in Norway at the time,” says Mags.

“Pop music was one hour a day, whereas in Britain every week there was a new band, something exciting going on. British pop records only got played in Norway after they were already big. It took ages for things to happen.

“We came over with songs but with no sound. It was just the three of us and a strong belief in the material and each other.”

Now they’re bowing out with the Ending on a High Note tour. But with so many recordings, how do they choose what to play across 15 countries in four continents?

“Choosing the songs for a tour is always a struggle,” says Pal.

“You’re guided by which ones were hits in whichever country you’re playing, but the more albums you make the harder it gets!”

Whatever they choose to perform on November 26 at the BIC, you’re not going to get a manufactured boyband from an era which is (wrongly, in my opinion) remembered for valuing form over function.

You’re going to hear one of the most enduring acts to emerge from a decade which deserves more credit for originality than it’s popularly given. You’re going to see three men who’ve known each other since they were boys and who honed their skills in suburban basements in a country not known for giving us world-beating pop bands.

You’re going to experience a unique blend of Scandanavian melancholy and pure, shining, pop music.

You’re going to love it. Trust me.

And don’t forget your stopwatch.