Manny Pacquiao's theme tune

The Scottish-Filipino band Rag Rag seems destined for fame and fortune after recording a theme song with the boxing champion.

The most unlikely moment of a surreal year for the Ragrag brothers occurred in one of the world's great recording studios in March. They were in Los Angeles laying down a track with one of the world's greatest boxers, Manny Pacquiao, when one of America's top R&B stars interrupted them. "So we're teaching Manny his bit," recalls Chris Ragrag, "and next minute who comes in? Chris Brown comes in: 'Manny, sorry to disturb you guys, I was listening; that's a nice song, man.' He was there for two minutes, but nobody bothered with him, because we were trying to get the song done. The funny thing, he goes out, and after two or three minutes Manny says to me: 'Who was that guy?'"

The globally popular Pacquiao deals with fawning celebrities on a regular basis, but for the Ragrag brothers, such encounters are freakishly novel. Due to a curious series of events, Glen, Chris and Mike - plus honorary Ragrag Norbert "Ace" Gayanilo - have recorded the world welterweight boxing champion's new theme song, Fighting Pride, which should get its first airing when he enters the ring against Antonio Margarito in November. That will be quite a moment for the London-based quartet, who usually play cover versions of popular hits at parties and weddings.

"The whole thing has just been like a big dream," agrees the lead singer, Glen, who has gathered his bandmates together in a west London bar to talk about Las Vegas, Los Angeles and beyond. "I'm like 'Wake me up, man! Somebody pinch me!'" In truth, they've had glimpses of the limelight before. Sons of a Filipino musician, the three Ragrag brothers were originally taken to Glasgow by their Scottish mother.

This was in 1990, and by the end of the decade they were travelling to London to play their first ever show at a Filipino festival. There they were spotted the promoter of a concert to be given by by the hugely popular Filipina singer Sharon Cuneta, who was looking for a local support act. The band's second live gig would be at the 12,000-seat Wembley Arena, and they added a new guitarist days before. "We came down to London, Norbert met us, we went straight to this church and his pastor gave us a space to rehearse," recalls Glen. "We were there for three whole nights before the concert."

The show went well (although "we didn't get paid for about two years", says Glen), and the brothers moved to London, with Gayanilo now a permanent member. From the heights of Wembley they gradually evolved into a covers band "to make some money", but also secured a useful job on the Filipino television channel TFC, as the regular backing band for any pop stars passing through London. Things moved up a gear when the Filipina singer Madonna Decena reached the semi-finals of the popular TV show Britain's Got Talent in 2008, and took them on her subsequent tour. Decena and Nugroovz - as they were then called - reached Manchester just as Pacquiao arrived to fight the local boy Ricky Hatton, and the Filipino's victory proved so inspiring that the band began work on their first original song in years.

"We're having breakfast, and we're at the piano, so we tried this song," recalls drummer Mike. "We more or less finished it in the van," says Gayanilo. The original version of Fighting Pride was "quite jazzy", remembers the keyboardist Chris. But then fate intervened: Mike's computer crashed and the recording was lost. This blow proved to be a blessing, as their next attempt was based around a more satisfying guitar riff from Norbert. "It was like a love song before," says Chris. "Now it's more like a fight."

Meanwhile, Decena, who had been introduced to Pacquiao during that Manchester visit, was summoned to Las Vegas to perform at the after-party for his next fight. On arrival she was informed that a backing band would be required, so phoned Chris, at 5.30am. The band were soon in Vegas too, and enjoyed a first taste of the A-list world as they and Decena prepared to visit the boxer. "We're taking pictures of these sports cars passing, and big limousines," recalls Glen, "then the biggest limo you've ever seen actually stops in front of us. So we're like 'Come on take a picture, man, quick, quick.' Next minute, the driver comes out: 'Madonna and the band?' and we all got the big eyes. 'This is our pick-up?'" Chris, the band's business brain, did the calculations. "Eleven televisions: I had to count them. It was a 36-seater."

Pacquiao was rehearsing with his own band, and during a break the gregarious Glen asked if Nugroovz could play Fighting Pride to him. It was a nervous moment as he sat "listening, eating, with some other people", recalls the singer, but they got the boxer's seal of approval and were asked to play it at the after-party. Before that performance Nugroovz were also fixed up with two paid gigs at the plush Mandalay Bay hotel, and offered a choice of equipment they still marvel at ("Mike's like 'This is the drum of my dreams!'" laughs Chris). Those gigs caused a near riot among the hordes of visiting Filipino fans, and a demo CD of Fighting Pride soon became a hot property in the Pacquiao camp. The idea of Nugroovz' song becoming Manny's new walk-out music gathered momentum.

The band returned to their old lives and things went quiet for a few months. Then, in March they were flown to Las Vegas's famous Studio at the Palms to meet a hotshot producer known only as Animal, who "used to work with Mary J Blige and Jay-Z", says Glen. "We never found out his real name." Having completed that recording, with added R&B elements from Animal and an extra vocal from Decena, they flew to LA to meet Pacquaio, who had agreed to appear on the song. The band wrote a spoken-word introduction based on his traditional pre-fight prayer, and the subsequent session, at the Record Plant, was perhaps the highlight of their whirlwind few months.

"We didn't realise this was the best recording studio in the world," says Chris. "So Chris Brown is in Studio A, we're in B. We got our own lounge upstairs with a 50-inch TV, four types of menu, a gym, a sauna. I go to the toilet and pass Studio C: it's Jamie Foxx." It was tempting to hang around but "the studio cost like $2,000 [Dh7,400] an hour", says Chris, so they worked quickly, ignored Brown and headed home. Six months on and with the completed song out as a single, the Ragrags are in a state of flux as they survey the months ahead. Do they continue as before or chase the dream?

The decade-old band have already been persuaded to change their name to Rag Rag, despite building a solid reputation as Nugroovz. Fighting Pride's debut at Pacquiao's next fight should raise their profile and Glen admits the band are working on new material to "ride the wave". But then, brother Mike is more circumspect about that bout. "If they use it and he loses," he says, "they're going to blame us!"

Whatever happens next, having already swapped weddings for Wembley, the Ragrag boys should be well prepared. They won't be throwing in the towel.