Joanna MacGregor: A pianist in concert with the present

Nestled next to its namesake hospital, St Bartholomew the Great in Smithfield is London's oldest and arguably most atmospheric parish church. Hogarth was baptised under its Norman arches; Benjamin Franklin served a year as a printer in its chapel. Having survived the Great Fire - and the scene where Hugh Grant gets hitched in Four Weddings and a Funeral - St Barts will soon be privy to Cityscapes, a multimedia snapshot of the City of London. Featuring a poet, a pianist, a saxophonist and two photographers-cum-visual artists, the project - especially commissioned for the City of London Festival - is the kind of collaborative, semi-improvisatory work anathema to most classical musicians. Joanna MacGregor isn't one of them. As far as the internationally renowned concert pianist is concerned, Cityscapes is another opportunity to push classical music into new territories.

Last month 42-year-old MacGregor played the Barbican with Quinteto Piazzolla, lending a typically inventive backdrop to the raw, urban strains of Argentinian tango. Before that she took Ultramarine, a recital accompanied by live video images, to Colombia. At the end of June she'll perform Gribben at the West Cork Chamber Music Festival; in July she's in Sydney, applying her astounding technique to everyone from Ligeti and Messiaen to Bach, Beethoven and Bartok. Oh, and come November she'll be touring the UK with jazz saxophonist Andy Sheppard (her collaborator in Cityscapes), tabla player Aref Durvesh and the Britten Sinfonia, an orchestra where she recently took up the post of Associate Artistic Director. With MacGregor conducting from both podium and piano (she made her conducting debut in 2001), they will perform a rearranged version (hers) of Bach's The Art of Fugue. Durvesh and Sheppard will improvise an ending to the piece Bach famously omitted to finish.

"I'm really interested in making orchestras come together through improvisation," says the Brighton-based MacGregor, sitting in a wine bar off Piccadilly. A reluctant interviewee, she's given up an hour between an Arts Council meeting and a gig in Soho featuring one of her favourite acts, lauded Australian jazz trio The Necks. "I've played Bach since I was a little girl," she adds. "I can't let a day go by without playing him. He's so witty and secretive and funny and mathematical and brilliant." MacGregor's passion for the established piano repertoire is as great as her love of jazz and new music; she is as likely to be found playing with leading British orchestras, with the Philharmonics and Symphonies of cities such as New York and Berlin, as she is interacting - and improvising - with performers from other genres and mediums. The fact that she also composes, conducts, lectures, runs her own record label, writes piano books for children and sits on all manner of boards is indicative of a freewheeling aesthetic that was nurtured, she says, in childhood.

The eldest daughter of a lay preacher who eschewed television, took his family to gospel church and championed home tutoring, MacGregor didn't attend school until the age of eleven. Her mother, a former music student, steered her through classical, jazz, gospel and even honky-tonk on the family Steinway in Willesden, North London. Blessed with a natural, instinctive talent, she didn't have a formal music lesson until she was 18. "As I get older I realise that start has made me rather, well, different. It set down a tremendous template for the rest of my life. I grew up believing the piano is a great instrument because you can play everything on it."

MacGregor's 2002 album, Play, which was nominated alongside pop and rock acts for the Mercury Music Prize and released on her own label, SoundCircus, is a case in point. With barely a pause, Astor Piazzolla's "Libertango" is followed by the delicate early Baroque of William Byrd's "Hughe Ashton's Ground"; Ligeti and Bach rub shoulders with Conlon Nancarrow, John Cage and Talvin Singh. As with all the eclectic programmes MacGregor devises and performs, continuity and similarity is vital - even if it doesn't always look that way on paper. "I enjoy making the connections and taking people down new paths," she says. "My favourite composers tend to be great improvisers as well as great players. It doesn't matter whether they're contemporary or classical."

"Bach, Beethoven and Mozart's time was much more chaotic and energetic than it is today. There was a hands-on approach that allowed them to run their own orchestras, to do everything. Now, as soon as you turn 16, it's like, OK, what are you going to be? Performer? Composer? Conductor?" For MacGregor, a proper musician is a multi-tasking one. "You don't rely on a great army of people to spoon-feed you. Today's classical-music world is very self-defeating. I love the fact that all these corporations are falling apart. They're sinking millions into acts and getting it wrong. And I know why - it's not about the music, or the audiences. So in many ways this is a very encouraging time."

Cityscapes will let MacGregor indulge her hunger for new experiences and demonstrate just how amenable the piano is to a host of musical places and emotions in a short (90-minute) space of time. Despite having worked with her collaborators - Sheppard, the poet Bernardine Evaristo and the photographers Fabian Monheim and Thomas Napper - for nearly a year, she sees the improvisation as vital. Sheppard and MacGregor have worked together on numerous crossover projects, often deciding just five minutes beforehand to start in, say, F minor or - true to the essence of jazz improv - not discussing what they intend to play at all. This time around, they will meet at St Barts, MacGregor's favourite London church, for a day or two's rehearsal with the other artists (Evaristo has been keeping a diary; the photographers' digital projections will respond to the performers' actions) and field recordings made by Sheppard over the past few months.

"We've taken modern London sounds, such as building-sites, the march of feet over London Bridge, the chop of the butcher's block in Smithfield Market, and fashioned them into rhythmic soundtracks," MacGregor says. "Andy and I will develop them with the live poetry and visuals." MacGregor will start off on a grand piano in the centre of the church, exploiting the venue's space, height and "ancient calm", before moving around a series of strategically placed keyboards and inviting the audience to move along with her.

"Joanna is quite fearless, especially for a concert pianist," Sheppard tells me later. "We jazz musicians are used to thinking on our feet, which a lot of classical musicians are very wary of. Joanna has no problem at all. In fact, the edgier, the better."

Still, it bothers her when purists complain that she is popularising the early classical composers, or concentrating too hard on the difficult, modern ones. "For a start," she says, "there is really no difference between Bach and Mozart's financial and creative struggles and the ones most living composers face today. They certainly have the same maverick qualities. It's only the telescope of time that makes them look all divine and God-given."

"Difficult" contemporary music just needs to be listened to in a different way: "These young composers are telling you about their past but they're using a contemporary, individual language." And just as MacGregor's oeuvre combines the historical and modern, so will her performance in Cityscapes. "The real star is the City of London," she insists. Which is, of course, debatable.

'Cityscapes' at St Bartholomew the Great, Cloth Fair, London EC1 today and tomorrow, 6.30pm and 8.30pm, and, with Andy Sheppard, at the Spitz, 109 Commercial Street, London E1, Wednesday, 9pm (0845 120 7502; www.colf.org)