Colin Tilney, harpsichord, City Hall, April 3 THERE are two problems with harpsichords. The first is that they have a tendency to sound, as Thomas Beecham put it, like two skeletons copulating on a tin roof. The second is that the bulk of their repertoire is from the baroque period, which has its ownspecial problems. Baroque music suffers because there is no continuity of tradition. People have been performing Beethoven continuously since he was published, so there is a fund of experience - some of it admittedly bad - that a performer can draw on. Baroque, on the other hand, was virtually unheard in the 19th century and has only been revived since the last world war. Accordingly, if a baroque performer wants to know about style, he or she has to research the answers, becoming almost as much musicologist as musical. The problem is exacerbated by the fact that baroque composers such as Bach did not write all the notes into the score. The basic outline was put down and the ornamentation was often left to the performer. It was quite possible to hear a Bach suite by two different performers and both times encounter something quite different. A Beethoven sonata is, by contrast, written out so this problem - or opportunity depending on your viewpoint - does not exist. Enter harpsichordist Colin Tilney. Tilney is outstanding. His technique beggars description and he combines it with just the right kind of elan to banish the skeletons from the roof. His phrasing and tempo are a joy, he communicates naturally with his audience and his playing has a compelling individuality. He is one of the few players of any instrument who could be recognised from a recording after the first few bars. They don't come any better than that. His rendition of Bach's English Suite No. 6 deserves special mention. The work is technically demanding and well-illustrates Bach's usual habit of leaving the bulk of the ornamentation to the performer. In the sarabande Tilney's ornamentation was exquisite, with a beautiful symmetry and logic. He salted the second gavotte perfectly with trills and wit - on the repeat of the gavotte some of his ornamentation changed, catching his audience unawares and providing fresh delight with every bar. The final gigue went faultlessly at a pace that crackled and led you inadvertently to hold your breath. In short, this concert was an event.