''Mozart played on new instruments, and that is the course which we have also chosen'' writes Jos van Immerseel in defence of his use of a fortepiano built in Brussels by Claude Kelekom as recently as 1978—its model, a 1788 Augsburg instrument by J. A. Stein. I thought its middle register a bit too heavy for the top when involved in Alberti-type accompaniment, as notably at the end of the variations. But on CD (the only version I've heard) the sonority is generously full and clear and lifelike, with resonance enriched by what sounds like an uncommonly responsive sustaining pedal. Jos van Immerseel's interpretative approach is best summarized by his remark that Mozart's genius ''does not live in busts, tombstones, dusty books or boring conferences''. In other words, despite recourse to an eighteenth-century-type sound-world, he is no dessicated academic but someone dedicated to what he calls ''creative handling of the music''. Trying to emphasize the improvisatory nature of the Fantasias, in contrast to the ''mundane character'' (his own words) of the Paris variations, he indulges in elasticity of pulse that now and again might even seem excessive in Chopin. I also question some of his rubato in the succulent middle section of the sonata's slow movement. Faster tempo fortunately brings much more rhythmic discipline, though stricter attention to the composer's own dynamic markings would not have come amiss in the sonata's finale. I thought him most truly stylish when most 'mundane', i.e. in the Ah, vous dirai-je, maman? Variations, where temptations to self-indulgence are fewer.'