The concert craze has only just begun

But one aspect of this development, arguably its most important, has barely merited comment. A state-of-the-art 420-seat auditorium, designed by Dixon Jones, architects of the Royal Opera House, and described by the site's visionary developer Peter Millican as "a jewel box", opens with a five-day festival in October.

Understandably it's being marketed as a "music venue", embracing everything from bongo to bass viol. But those in the know scoff at such weasel words. The fact that the Orchestra of the Age of Enlightenment and the London Sinfonietta will have their homes here gives it away. To state plainly: London is about to get a new concert hall.

This takes some digesting. Classical music is supposed to be struggling to find audiences. The capital already boasts several excellent venues. Wigmore Hall (550 seats) and Queen Elizabeth Hall (900 seats) already specialise in recitals of the kind promised at King's Place, where a week of Fauré and another of Haydn opera are planned for its inaugural season. It's hardly easy listening.

Were that not surprise enough, another new venue will open in 2010, Milton Court, at the Guildhall School of Music and Drama near Moorgate, providing its neighbouring partner, the Barbican, with further concert space. Cadogan Hall (850 seats), off Sloane Square, only came on the scene in 2000 and is steadily gaining ground. The superb LSO St Luke's (372 seats), has already made a huge impact in its brief life.

All this without even mentioning London's major venues: the revitalised Royal Festival Hall (2,900 seats) now riding high, the eclectic Barbican (1,949 seats) and, during the Proms, the Royal Albert Hall (6,000 seats).

What is going on? By tacit consensus classical music has undergone a quiet revolution. After nearly four decades of identity crisis, it has shed its withered old skins and emerged new, bold and exciting. There's never been a shortage of seriousness or quality, but only Pavarotti's top Cs or the latest diva tantrums have attracted wider interest.

Concert halls now, as often as not, are full. The Southbank's Barenboim-Beethoven extravaganza, with 22,000 tickets sold, was in a category of its own. The string of hit concerts requiring tough listening that London has seen in recent weeks - Gergiev and the LSO performing Mahler at the Barbican, Jurowski conducting Shostakovich with the LPO, and the Philharmonia in Messiaen at the Festival Hall - is typical. Before we know it they'll be queuing round the block for Boulez.

Sir Nicholas Kenyon, who has been attending London concerts since the 1970s, first as a music critic, then as head of the Proms for a decade and now as managing director of the Barbican, is categorical about the change: "I can't remember any time when the sense of serious music being important has been like this. When I first arrived in London, you still got meat-and-two-veg concerts. You couldn't tell one orchestra from another."

Things started to shift in the 1980s and early 1990s with "events" such as Roger Norrington's Haydn Experience, Klaus Tennstedt's Mahler cycle or the LSO's Tender is the North. But only now is this new kind of programming really coming to fruition.

It helps that buildings have improved, the coffee is better, the stuffy, antiquarian atmosphere banished. Kenyon continues: "Classical music, until recently, was a closed shop with a know-what-we-like audience. Now it's far more inclusive - please note, Margaret Hodge - experimental, volatile, broadminded. The audience pool is bigger, even if they go to fewer things."

Marshall Marcus, head of music at the Southbank, agrees. Having had a long career as a concert violinist, he also sees the players' viewpoint: "Artists want to come to London. It's a happening city, a place of enquiry - much as it was in the 18th century for people like Haydn. The rivals in terms of vitality are not Berlin or New York but Bahrain, Delhi, Shanghai. It's inexplicable to me that London still has this magnetic power but it does."

Ticket sales bear this out. The Southbank recently announced a residency by the dazzling Simón Bolívar Youth Orchestra of Venezuela. Sixty per cent of seats have already been snapped up for concerts more than a year away. Similarly at the Barbican, the Great Performers season last year achieved 88 per cent capacity, its largest ever, with nearly 70,000 attendance.

As for the prejudice that classical audiences are old, you only have to look to see this is no longer true. In the thronging bar at the interval of the LPO/Jurowski concert, I estimated one in five to be grey-headed. This cannot wholly be accounted for by increased use of hair dye.

Who are these audiences? The anoraks are still there with their sandwich boxes and their encyclopaedic knowledge of Kochel numbers and historic recordings. But now fashionable urban young come in lively gaggles. Many are music students or foreign tourists or both. Women book club members, formerly habitual theatre goers, now drift towards the Festival Hall because they've sensed a buzz.

Concerts are also a favourite rendezvous for upmarket online daters: safe, populated, and if the going gets tough you don't even need to speak. Several halls have their own networking clubs. Equally, going to a concert on your own is entirely stigma-free.

Last week I sat next to an Estonian medical student, there to hear Shostakovich. Two rows in front was Vivienne Westwood with a young man. A couple with his 'n' hers pony tails sat alongside a gay couple. This may be anecdotal but how else can we judge cultural diversity?

Opera merits a separate discussion. But in this context we should note that the Royal Opera House has just commissioned research from Dunnhumby, one of the world's leading customer data analysis companies, part-owned by Tesco, who developed the Clubcard.

Dunnhumby is sure to have provided the ROH with powerful information, the full details of which are not yet released. But one headline finding is that whereas young and old go to the opera, those in their thirties and forties stay away.

There is no real mystery to this: many prefer concerts at this stage of life. They're cheaper and finish at least an hour earlier - crucial if you're paying a babysitter or are short of sleep.

Inevitably there are casualties in this brave new, bustling musical world. The taste for big events means stand-alone concerts, however good, may be neglected. St John's Smith Square is now dark several nights a year. Blackheath Concert Halls has struggled - and it has not been plain sailing for King's Place either, which has already seen one artistic director come and go.

Even Wigmore Hall, with its fiercely loyal audiences and perfect acoustic, must be wary of the prospect of a King's Cross rival. But John Gilhooly, its director, reports record sales and a £130,000 surplus last year with the current season expected to be up by at least five per cent.

He is buoyant, but adds a word of caution: "This city remains one of the most competitive music environments in the world, and achieving this success is not easy." But London has also, for those of us who live for music, never been more rewarding or exhilarating.