N-Dubz, Brighton Centre

N-Dubz's music is throwaway post-grime cheese-pop aimed at fans aged between 10 and 20, max. I've been writing a rearguard action for electronic pop in the pompously self-assured court of rock for more than a decade so I arrived at the Brighton Centre ready to sponge it up.

Almost everyone present was a teenager or in loco parentis. Despite an expected sell-out the place was two-thirds to three-quarters full. This may be because N-Dubz peaked about two years ago, or it may be that Boho old Brighton is just not N-Dubz's core territory. Either way, much of their target audience has, perhaps, moved on to UK gangster rappers such as Giggs, who make N-Dubz seem innocent by comparison.

They've certainly made an effort. The show begins with a film on big screens behind and either side of the stage. In it N-Dubz, a trio consisting of north-London lads Dino "Dappy" Contostavlos and Richard "Fazer" Rawson alongside Dappy's cousin Tula "Tulisa" Contostavlos, face down a whisky-drinking Cockney gangster who appears to be extorting them. It's difficult to tell what's going on, so thick is the air with teen screams of adoration. Then Fazer drinks off the gangster's whisky and the trio do a runner on mopeds - Tulisa's is pink - chased by henchmen in cars. The stage lights up. It's a two-level affair designed to look like a garage, replete with custom hub-caps, a petrol pump and parts of a Mini. On the upper level stand the mopeds and - bingo! - there are N-Dubz in silhouette. Shrieks almost lift the roof off the Brighton Centre. It's the best moment of the night. Dappy and Fazer are in black hoodies and Tulisa in a body-hugging black outfit. Beneath them in the shadows a full band plays. There are six dancers, three male and three female. And then there's the music.

Let's get this out of the way early: N-Dubz's music was relentlessly horrid. Fronted by sub-grime rapping and the pauper's Beyoncé shtick of Tulisa, it's a saccharine car crash of amped-up Black Eyed Peas-ish Euro-trash, bombastic R&B flavours and - weirdest of all - a germ-free soft-rock sheen redolent of Bon Jovi at their most overproduced. It's the music both George Orwell and Aldous Huxley predicted for their dystopias, vapid content-free aspirational anthems. The crowd, however, lapped it up, especially when five giant bursts of flame roared up from the stage during the 2009 hit "Playing With Fire". Soon it was time for a change of scene.

While the stage was dark the band jammed hideous hair metal then, hey presto, we're in an N-Dubz version of a bordello-cum-speakeasy. This means that all the males wear trilby hats and are dressed like 2-Tone fans circa 1979 and all the ladies are in basques and lacy mini-skirts. Dappy doesn't wear the full ensemble. On his head he has what appears to be a giant liquorice-flavoured condom. I know, I know, it's a bit old knocking Dappy's hats. They're his visual gimmick and harmless enough. This does not mean that they look any better.

So it all goes Guys and Dolls for a bit as they play "Girls" and "Toot It and Boot It" from their latest album Love. Live. Life. Then the video screen shows a clip of Tulisa aged 11 singing "My Name is Tallulah" from Bugsy Malone. Sitting on a mini-grand ostensibly played by Fazer, she asks us if we'd like to hear her 2011 version and lays it down a capella, "My Name is Tulisa". It's a witty move but the crowd simply doesn't get it and falls quiet, bemused. "I'm only kidding," says Tulisa, wrapping up quickly, "How ya feelin', Brighton? Over-18s had a drink? Under-18s had a Red Bull?" And she brings on her pal Lady Ny who has legs so long they look like they've been Photoshopped, and the pair sing "Love Sick". The crowd wakes up and screams again.

The friend I came with has to leave for a while. He can't take it any more. Neither of us is 15. This is a problem

The scene changes once more, to a spectacle of lasers and LED, the backscreen dripping green data like in The Matrix. This time the dancers are dressed a wee bit like Star Wars stormtroopers with their hoods off, Tulisa is in a rubberised mini-dress and the boys wear what can only be described as futurist hoody gear. The trio appear hydraulically out of the stage into cones of green laser light. The ambition and theatre of this show is admirable. If only they could match it with a song that will carry over the decades.

Five Star spring to mind. They were massive a long time ago, a family affair that sprung from the urban R&B pop milieu. One of them dated Eddie Murphy. They didn't have any songs. No one remembers them. N-Dubz play more of theirs - "Morning Star", "Say it's Over" and "Cold Shoulder". The latter would be a fun hi-NRG anthem if it was allowed to breathe rather than being straitjacketed in sonic candy that makes my ears flinch.

The friend I came with has to leave for a while. He can't take it any more. Neither of us is 15. This is a problem but I strongly suspect my 15-year-old self would have disliked the music equally. Although I might have enjoyed the giant sparklers that spring from the stage in the final sequence. This time N-Dubz have gone "old school". This means they wear colouful tracksuits, except for Tulisa who wears a tracksuit top and another mini-skirt. She's an attractive girl but not an especially natural entertainer. She makes a comment about imagining her wearing this outfit in Heat magazine. Just imagine. Girls shriek. I feel like an alien who has popped in from another universe. I am an alien who has popped in from another universe.

N-Dubz perform a medley of older material as if it were from decades ago. It probably seems that way to the tweens behind me. To my right a lone middle-aged lady in a black lacy bra top jumps up and down and yells with glee. Perhaps her kids are here but I don't see them. To relate so ecstatically to this music in middle age strikes me as a portent of mental-health issues. The older fare is slightly more percussive and has stronger hints of the band's London hip-hop roots. It's a tiny bit better, in fact. With it everything reaches a climax and I run for the doors.

Everyone's had a fabulous time and I can only commend N-Dubz's desire to put their money where their mouth is, investing in a smashing show. I must also admit that they floored me. We music journos are perversely proud of our eccentrically catholic musical tastes but, while they make rather good pop stars, N-Dubz's music, previously absorbed in manageable bite-size chunks on TV and radio, most certainly defeated me.

• Find N-Dubz on Amazon

Watch the video for 'Girls'