When Stephen Hough plays at the Queen Elizabeth Hall on Sunday, he'll insert an obscure atonal work between two favourites by Mendelssohn and Beethoven. This is standard Hough behaviour, and in no way perverse. The work in question - Ben Weber's Fantasia Variations - "glances back to Brahms through the lens of the Second Vienna School's 12-note technique". But Hough is as likely to yoke Grieg with Schoenberg, Copland with Mozart. It's the element of surprise and illumination, inspired, he says, by the chronologically-chaotic Frick Museum in New York. "Just as it's refreshing not to be in a series of huge rooms full of Van Dycks, so it's a relief to hear things in fresh juxtapositions. This matters greatly today, when we've heard everything before."

Well, not quite everything: this indefatigable musical explorer is doing his best to fill in the gaps, and winning a string of Gramophone Awards in the process. It's in large part thanks to him that Hummel, Scharwenka and Mompou are again on the musical map; if the British composer York Bowen still isn't, at least Hough has given him the boost of a fine CD.

The Austrian pianist and composer Johann Nepomuk Hummel was a contemporary of Mozart and Beethoven whose works have long been consigned to the cupboard. Hough's CD was the result of an invitation from Chandos to record something - anything - by John Field. Hough decided that Field wasn't interesting enough (well, he was only the man who invented the Nocturne form), but that the slightly earlier Hummel was.

"His historical importance is enormous. He was the bridge between the classical and Romantic styles, and absolutely everyone played him, all the great pianist-composers like Liszt and the Schumanns carried him in their heads. Without Hummel, Chopin's achievement would have been unthinkable."

Fine, but what about Hummel's music two centuries on? "Um, there you're on thinner ground." Hough knits his brow. "The pieces are charming, there are moments of beauty... but I don't think there are any moments of greatness."

When asked by Chandos to do another Hummel CD, Hough replied that one was enough. "If you're playing unusual music, you need to know its strengths and weaknesses. You should only play the best of it."

What about the Scharwenka concerto for which he's just won an award? "Ah, that's much more interesting to listen to. It's a big, glittering Romantic concerto, which loses nothing to Tchaikovsky's First, but it came when the Romantic period was over, so it's been overlooked. It's fun to play - it's got applause woven into it."

On which point he's glad to elaborate. "There are certain pieces - many Romantic pieces - where the applause is the unwritten final bar. They are designed to have people leaping out of their seats and cheering, and sound incomplete without it. If you just get the CD clicking off, you miss the point. The challenge of making this music work in the recording studio is an interesting one."

With the work of the equally forgotten Federico Mompou, however, even the presence of a microphone can feel intrusive, let alone an audience and its applause. "It's not concert music, it's not even salon music - it's solitary, a one-to-one communion, like meeting someone on the Internet." And Hough is making a bigger deal out of this gentle Catalan composer, who spent most of his life in Barcelona and died there in 1987. He's written memorably about Mompou's work, in an essay identifying this as "the music of evaporation... The printed page seems to have faded, as if the bar-lines, time-signatures, key-signatures and even the notes themselves have disappeared."

One of Mompou's largest collections of pieces - Musica callada ("Silent music") - might just fit on to another CD which Hough may, or may not, record. If he doesn't, it will be because he can't stand the idea of producing the "complete" anything, and because there's a great deal more on his immediate agenda.

"One of the most exciting things a performer can do is collaborate with a composer," he says. He's done so with great success with Juilliard fellow- student Lowell Liebermann; he's now contemplating a project that sounds at first blush bizarre. Until the arrival of the Urtext and the period- authenticity craze, he points out, concerto cadenzas were usually composed (if not actually improvised) in the style of the time in which they were performed: he's currently commissioning new cadenzas for all Mozart's piano concertos. While Elliott Carter and Peter Maxwell Davies declined his challenge, a string of others (led by John Corigliano) have agreed to try.

One thinks of the critical flak Gidon Kremer received when he introduced new cadenzas by Alfred Schnittke into the Beethoven Violin Concerto: isn't Hough afraid of attracting the same kind of criticism? "Mozart is like Shakespeare," he says. "He's universal enough to stand up to a lot of different approaches. This should lead to some interesting compositions."

He says this as though he were a scientist describing an experiment. His playing may combine fire with fastidious perfectionism - the late Shura Cherkassky regarded him as his natural successor - but he seems devoid of arrogance.

He looks back gratefully to the support of his parents, who realised that the pounds 5 piano they bought their five-year-old son had unlocked an extraordinary talent, but who carefully shielded him from a surfeit of "young Mozart" publicity. He dates the start of his obsession with "unusual music" to the point when, as an impressionable adolescent, he read Harold Schonberg's book The Great Pianists. But he attributes the strength of that obsession to the effect of the first record his parents bought him. "It was called Keyboard Giants of the Past, and it had things by people like Paderewski, Rachmaninov and Cortot. I was formed by the spirit of 78s."

In fact he's so articulate, analytical and laid-back about things that I begin to sense a mystery, which is only dispelled when he refers en passant to his Roman Catholic faith. That is the religion of certainty. A useful rock, if you can accept it intellectually, on which to base the uncertain life of a globe-trotting pianist.

Recital: 3.30pm Sun, QEH, SBC, London SE1 (0171-960 4242)