Between them, tonight's bands have been together 81 years, released 34 albums and had 24 members. These are amazing statistics, even before you get to Deep Purple's 38-piece orchestra, or Cheap Trick axeman Rick Nielsen's dozen or so guitars...

Illinois's finest have the tough task of opening proceedings as people are still planting their bums on the O2's comfy, cushioned seats. Cheap Trick can be relied upon to raise the roof of theatre-sized venues across the land (this writer saw them level Dublin's Olympia in June), but it's a long time since they headlined in arenas - and never in the UK. So, for the first few songs - 'Ain't That a Shame', 'Hello There', 'California Man' - the London audience simply nod politely. The reaction is similar to that which you might expect for an unknown support act. Fortunately, the US four-piece are used to such indignities (they were reduced to opening for Mötley Crüe and Ratt in the 1980s, remember), and they plough on. Eventually, the evergreen 'I Want You to Want Me' gets all but the most partisan Purple fanatic onside, while unofficial leader Nielsen's trademark dry wit has the whole arena chuckling into its overpriced beer.

Introducing 'If You Want My Love', the eccentric legend notes that the song appeared on the soundtrack of "one of the finest movies of the last 25 years", a flick that "should have been up for a major award". The "great, great, great movie" Nielsen's referring to? 'Joe Dirt', starring David Spade. Not many American rock musicians are this amusing or self-deprecating, but then not many American rock musicians know they're sitting on an arsenal of gems that could decapitate an X Factor judge at 40 paces. 'Need Your Love', 'Surrender' and 'Dream Police' certainly fit this bill, as does recent track 'Sick Man Of Europe'. Even the Bob Mitchell and Nick Graham-penned '80s über-ballad 'The Flame' ("It was number one everywhere except here," smirks Nielsen) is a winner tonight. If there's a criticism of the hour-long romp, it's that the time spent playing three covers (as well as the Fats Domino and Move tunes, we get the Beatles' 'Magical Mystery Tour') could have gone to, say, 'He's A Whore', 'Southern Girls' and 'She's Tight'. But a heavy, extended 'Gonna Raise Hell' - complete with Nielsen whipping out his famous five-necked Hamer - is a treat for aficionados and converts alike. Cheap Trick remain rock's best-kept secret, a band that continues to excite and inspire in their fifth decade together. The die-hards get it; now the Purple massive get it. Maybe one day the rest of the world will catch on.

On paper, the pairing of the Trick with Brit-metal dinosaurs Deep Purple might seem odd, but both sets of musicians cut their teeth on the swinging '60s. Of course, in Purple, only drummer Ian Paice endures from those formative years, due to the labyrinthine line-up changes that have seen the Hertford outfit go through five lead singers, four guitarists, three bassists and two keyboardists (in addition to splitting up for eight years and suing ex-frontman Rod Evans for launching a rival DP in 1980). The current crew have been on the go longer than most, with even 'new kid' organ maestro Don Airey having notched up close to a decade at the Hammond.

After a rousing orchestral intro, which stitches together snippets of some of Purple's biggest tunes like a decibel-abusing Frankenstein's monster, the veteran fivesome make their entrance. Despite Ian Gillan's voice not being what it once was - and by now most fans have accepted this - 'Highway Star' is a storming opener, with a head-banging cellist seemingly in agreement. Paicey drives 'Hard Lovin' Man' along, with Airey and guitarist Steve Morse giving it all they've got. The public may never fully accept Morse in Ritchie Blackmore's boots, but his playing ticks the boxes. 'Maybe I'm A Leo' drags a little, with the orchestra sitting on their hands during the solos, of which there are many. 'Strange Kind Of Woman', sax break and all, rekindles the affair, if you can forgive some Gillan screams that sound like Jimmy Savile yodelling from beyond the grave. So far, so old school, but here it comes, "the title track of our latest album", 'Rapture Of The Deep'. "It's merely five years old," sighs Gillan, who looks like he wouldn't mind joining the deserting fans at the bar."It's about... It's about a quarter past," he musters, glancing at his watch. Following a fine 'Woman From Tokyo', the next stretch of the set is frittered away on singing 'Happy Birthday' to bassist Roger Glover - who drops to his knees in apparently genuine appreciation - and accomplished but characterless Morse solos. The ensemble claw their way back with a wry 'Knocking At Your Back Door' ("It always makes my eyes water," winks Gillan), while 'Lazy' turns out to be the perfect marriage of group and orchestra. Conductor Steven Bentley-Klein screeches out a violin solo, duelling with Morse, while Gillan tortures a harmonica. It's brilliant.

'No One Came' leads into a superbly entertaining keyboard solo that takes in boogie-woogie, classical and 'Maybe It's Because I'm A Londoner', before the concert climaxes with heavy-duty renditions of 'Perfect Strangers', 'Space Truckin'' and 'Smoke On The Water', classic rock if ever there was some. For the encore, Purple wheel out the old blues standard 'Green Onions', Billy Joe Royal's 'Hush' - incorporating a masterful drum solo from the guv'nor Paice - and trusty hit single 'Black Night'. 

This evening's performance has reminded this writer of several things: firstly, that Cheap Trick deserve more, much more; secondly, that orchestral hook-ups can be a good idea in the right hands; and thirdly, that some hip, new rocker needs to start wearing a 'Listen To Deep Purple' t-shirt and get these guys cool again. Critics, historians and hardcore fans understand the band's value, but as far as the casual punter is concerned Black Sabbath invented heavy metal. No disrespect to Tony Iommi and co, but the Purps were already onto their third album when the Sabs were still mucking about in Midlands pubs.