The Polish virtuoso Krystian Zimerman is nothing if not unpredictable. For a start, you can never be sure he’ll turn up. Indeed, a cynic might suggest that his legendary status is enhanced by the frequency of his no-shows.

His presence at the advertised recital last night was the first of his pleasant surprises. The second came after his rendering of Bach’s Partita No2 in C minor. While he took his bows, a technician whisked the entire keyboard action out of his piano and inserted another in a matter of seconds. Zimerman is famous for modifying his instrument to his own specifications, but it’s not every day you see a Steinway transformed in front of your very eyes.

The point of it was immediately clear in the increased bloom of tone he could draw on for Beethoven’s last sonata, the visionary Op11 in C minor. But the rationale behind the emaciated Glenn Gould-like tone of his Bach keyboard was questionable. The Courante was as forceful as the Allemande was languid and elegiac, but neither was remotely Bachian. The impeccable voicing of his Sarabande equally evoked the calm, unruffled surface of a moonlit lake. But for any intimation of a courtly Baroque dance you’d have to look elsewhere, just as you’d be hard put to identify anything of the 18th century in the fast waltz he made of the movement labelled Rondeaux.

The question is: why reduce your vehicle to a hard-edged, pseudo-period instrument and then romanticise the music out of recognition?

Once endowed with a conventional piano, Zimerman proceeded to dazzle the audience in Beethoven, Brahms (Op119) and Szymanowski (Variations on a Polish Theme) with alternations of transcendental virtuosity and super-refined delicacy of touch. His tone in the treble register is pearly and luminescent, in the bass amply upholstered; even if the final Rhapsody of the Brahms set sounded like the swinging open of the Great Gate of Kiev, Zimerman’s tone is never unpleasantly clangorous.

His final surprise? Surely only a superstar could respond to a standing ovation by refusing an encore.