Chemical Brothers, Guildhall, Portsmouth<br></br>Pop Idol tour, Wembley Arena, London

It's always worth applying scepticism when anyone tells you that a now-defunct club was "legendary", but in the case of The Heavenly Social, it's justified. I first saw Ed Simons and Tom Rowlands wreckin' tha decks at the Social before it moved to Turnmills overkill, when it was still a Sunday night best-kept secret in a Marylebone pub basement. Back then, they were still operating under the stolen nom de stylus Dust Brothers, and had a joyous disrespect for generic boundaries, mixing the Manics into Grandmaster Flash into Flowered Up into Love Unlimited. In 2002, this doesn't seem at all unusual, but in the early-to-mid Nineties, they helped bring a new playfulness to a po-faced clubland.

Which is why it's surprising, in a way, that the Chemical Brothers make you work so hard for your pleasure. Tonight, they kick off with "Come With Us", the title track of the new album, which has the sort of Jurassically heavy beats which, amplified by a quadraphonic sound system, threatens to do the same sort of damage to central Portsmouth that Chris Morris's bouncing elephantiasis woman did. It's physically impressive (and oppressive), but it quickly becomes a monotonous thud. This is what you must endure, until Ed and Tom allow a sudden break in the cloud for an interlude of pure melody.

It's like your mum only letting you have jelly and ice cream after you've eaten up your greens, or telling you not to put your coat on until you leave the house because "you won't feel the benefit". It's a very English thing, this, a very Protestant thing.

Visually, The Chemicals themselves are nothing to write home about. Rowlands, the one who used to look like a yak, has had a haircut. Occasionally, Simons will hold a White Panther salute, or raise both hands in beatific supplication (then again, maybe he's asking "How are we getting away with this? Search me!"). The rest of the time, they pore over banks of possibly pointless LEDs, like those over which Lt Uhura and Scottie presided in Star Trek.

At a show like this, therefore, the most crucial person is the lighting director. I wasn't allowed to see the show in Birmingham the previous night because the venue didn't allow "the full production" (it was good enough for a couple of thousand paying punters, but not for a broadsheet critic). At first, the most fetching effects are the Guildhall's red neon No Smoking signs, and I'm at a loss to see what the Brummies were missing. Then a giant circular trampoline is lowered over the duo's heads, and a huge projection wall opens up behind them. Between them, they show kitschy robots, mathematical formulae, Blair Witch-style running-through-forests footage, and high-speed stroboscopic flashes (a Chemical Brothers show is not for epileptics).

It holds the attention, at least, during the cloudy, thudding passages until the shafts of light break through. There's little obvious improvisation (apart from what amounts to a cover version of New Order's "Temptation"), although the Bambaataa collaboration "It Began In Afrika", the jubilant "Block Rockin' Beats" and the anthemic "Hey Boy Hey Girl" send the place predictably berserk. If this place is a spaceship, she cannae take it, Cap'n.

There's a strange shimmer on the streets of Wembley. Is Empire Way paved with gold? On closer examination, it's the fallen glitter from a thousand plastic stetsons. Such party paraphernalia is only to be expected: the Pop Idol tour is a magnet for people who don't get out much.

They're also the people who don't buy records much. I'd wager that the last one most of them purchased was made by Hear'Say, and the one before that was "Candle In The Wind '97". The billboards advertising Gareth Gates' single urge us to "Be part of pop history", but in every sense that matters, this is the end of history. No amount of stats about Will Young's single being the biggest seller of all time can mask the fact that, with what amounts to a free six-month campaign of hour-long adverts on prime time ITV, he could have released a recording of his own bowel movements and still achieved the same.

All the millions who phoned in to vote for one or the other were defrauded of their pennies. The winner, nominally, was Young – the fat-tongued, canoe-chinned toff who kept quiet about his sexuality until the counting was done. But there are no losers. Each of the final 10 gets to perform on the arena tour, and gets a five-year contract. What more could they want?

Tonight's show starts with the supposed also-rans: Korben Nosurname ("Freedom"), Jessica Garlick ("Papa Don't Preach"), and Aaron Bailey, the Geordie train driver who seems to be a genuinely nice bloke. He sings "Walking In Memphis". You pray that he doesn't let his ASLEF membership lapse.

Laura Doherty performs "Son Of A Preacher Man". It's so unremarkable that, as only an occasional viewer, I wonder what the other 39,990 contestants must have been like. Even with a radio mic clamped to her face and a PA system the size of two bungalows, she's nigh inaudible. Rosie Ribbons sings "Everything I Do (I Do It For You)" in a voice that's flatter and deader than hedgehog roadkill, but she does have a great porn star name (and, y'know, give it five years or so). Hayley Evetts ("Made For Lovin' You"), at least, is already making other career moves: she tells us she's going to interview Britney on telly.

Presenter/judge Nikki Chapman, wearing a tight black leather Dress (which feels every bit as wrong as seeing your mum in... well, a tight black leather dress) introduces Zoe Birkett as "the most famous 16-year-old in the world". It's an odd concept of what "world" means, but Zoe is the first singer who can actually sing, the first performer who can actually perform, belting out "I Say A Little Prayer" with much gusto and character.

Darius Danesh is a very British kind of celebrity: the loser who doesn't know when he's beaten, the Eddie "The Eagle" Edwards of pop. He's pure cheese: cut his wrists and he'd bleed mozzarella. Nevertheless, as his intro film begins, the hysteria clicks up a notch. Yes, you can "feel the love in the room". If nothing else, Darius – now clean-shaven and short-haired – can work a crowd, gyrating through "It's Not Unusual" with a microphone stuck saucily into his waistband. At the end, paid Wembley workers throw underwear.

Then the hysteria really goes off the scale. Doctor Fox is drowned out by screams while introducing Gareth Gates. It's been said before, but Gates's story is almost too picaresquely perfect, a screenplay waiting to happen: the beautiful boy who can't speak, but can sing like an angel. He's as nervous as hell, (the anti-Darius) as he goes through the motions of "Unchained Melody", but no one cares: "Marry Me Gareth" banners are everywhere.

The response to the winner is muted: Will Young's rendition of "Light My Fire" gets the big budget pyros, but there are no "Marry Me Will" banners. "Thanks for making this song Number One" he says introducing "Evergreen", before the Top 40 has been officially announced. He may be stating the obvious, but it's also ungracious.

After an interval, the 10 return with a big band for a set of Rat Pack covers (where have we heard that idea recently?). Zoe, giving it plenty of Fabulous Baker Boys piano-writhing for "I've Got You Under My Skin", looks born to do it. By contrast, Gareth, swinging his arm like a dead branch through "Mack The Knife", could scarcely look less comfortable.

Darius, rocking the Connery in a white tux for "Let There Be Love", breaks off to reminisce. "Six months ago," he says, tears welling up, I'd never have thought I'd be here doing this." Didn't anyone copy him the schedule?

s.price@independent.co.uk

Pop Idol tour: NEC, Birmingham, today; MEN Arena, Manchester, Mon; SECC, Glasgow, Tue & Wed (0870 1633401)