Not one but two pianos filled the stage for this, the first instalment in Andras Schiff's three-year survey of the Beethoven sonatas. As he swapped instruments - and squeezed around them to take applause - we experienced two opposing aspects of his performing style.

It is puzzling that one of the piano's more intellectually driven advocates can, at times, make music that sounds so shapeless. This was most in evidence here in the first sonata, Op 2 No 1. It was played on a Steinway, by far the mellower of the two instruments, but came over as lumpen and relentless, with an impatient opening movement and little tenderness in the Adagio.

Yet, by the end of the programme, in the Op 7 Sonata in E flat, Schiff seemed a different player, shaping the second movement's mercurial changes of mood into one continuous arc, and coaxing out the countermelodies of the third. All this despite playing an instrument - a Bösendorfer this time - with a sound so jangly it could almost take your ears off.

The transformation had admittedly been gradual. In between, the Op 2 No 2 Sonata, played on the Bösendorfer, had brought out more of a sense of phrasing and shape, and the third, Op 2 No 3, for which Schiff switched back to the Steinway, had him contrasting lyricism and grand, thunderous outbursts in the slow movement, creating a real sense of tension.

How can Schiff's playing be so foursquare in one piece and so sensitive in another? Perhaps he lets himself be driven by harmony so much that the niceties of melodic detail and phrasing are sometimes passed over. The way in which, in the most thickly textured passages, he chooses to point up bass lines rather than emphasising more obvious material in the right hand seems to suggest this. Or maybe he just found the first sonata less interesting. At least his Beethoven series isn't going to be predictable.