David Briggs, St Michael Cornhill, London

Improvisation is often less spontaneous than it seems. Organists, who practise the art more than any other type of classically trained musician, often rely on stock procedures and situations that can be applied to any theme. It's a party trick, albeit very clever.

Thus David Briggs, organist emeritus of Gloucester Cathedral, made a symphony in several movements out of "London's Burning" and "London Bridge Is Falling Down", given to him in sealed envelopes before our eyes. The style was French Romantic, including an aerial scherzo on the second theme, a fugue on the first, and then a shattering toccata with "London Bridge" in the pedals. It brilliantly explored the colouristic range of the 63-stop, three-manual organ in St Michael Cornhill.

The church isn't big, but if you close your eyes, that powerful instrument sounds as if it has deep roots and plenty of space to spread. Three of Vierne's Pièces de fantaisie were sumptuous, the Prélude recalling Ravel's evocation of dawn in Daphnis and Chloë. With Bach's Prelude and Fugue in D major, BWV 532, Briggs thought of what might sound effective on that organ rather than vainly seeking a Baroque sound. The rhetorical opening positively bulged, and in the Fugue there was even a crescendo, something rarely heard in Bach these days.

The much smaller, two-manual organ in St Mary-at-Hill has recently been restored to something close to its original 1848 form. It was one of the first English organs with a pedal division able to cope with Bach's music. Even so, Bach's Toccata, Adagio and Fugue in C major was an unfortunate choice to open Guy Bovet's recital. The instrument seemed to suffer from tuning and cipher problems (pipes that wouldn't stop sounding). But Bovet's sense of rhythm left a lot to be desired, too, and he larded the Adagio with inappropriately operatic embellishments. He hardly made sense of Schumann's exquisite little Studies in canonic form, and even mangled Mendelssohn's rousing but straightforward Prelude and Fugue in C minor.

It was something of a relief when he ventured into the unknown with an improvisation on "Angels from the Realms of Glory", beginning with a sort of French Baroque bagpipe effect and ending like a fairground steam organ. If the intervening journey was unpredictable, it wasn't exactly coherent, and for some time Bovet seemed at a loss, playing the fool with perky dissonant contrasts, until he found refuge by broadening the tune in a quiet interlude.

Recitals at St Michael Cornhill today and on 21 and 28 July at 1.30pm