Ludus Danielis, Southwark Cathedral, London <!-- none onestar twostar threestar fourstar -->

Lords of Misrule, Feasts of Fools: these days, we're all too familiar, at the political level, with the spectacle of lunatics taking over the asylum. But the original concept, as practised in medieval England and France, was benign: a therapeutic turning of the tables in the social hierarchy, a letting-off of steam, a way of telling those in power that it's time they relinquished it.

The winter entertainment devised by young clerics at Beauvais Cathedral in the late 12th century was an unusually graceful variant on this theme: a music drama named Ludus Danielis (The Play of Daniel), based on the Old Testament story of the exiled prophet Daniel. The protagonist is summoned to Belshazzar's feast, thrown to the lions, and then prophesies the coming of the Messiah. The placing of ass's ears on King Darius is the revolutionary climax. Since the surviving manuscript of this drama consists of just one line of notes, it presents a fascinating musical challenge.

Enter the Harp Consort, whose severely cassocked appearance suits their role as latter-day monks. The harp in question is a dainty little thing strummed delicately by the group's leader, Andrew Lawrence-King, as he delivers his preamble. Southwark Cathedral forms a stage-set beyond compare, while on a dais in the middle of the chancel stands an elongated red-lacquer chair, like a ladder to heaven: the sole prop that the director Akemi Horie, who specialises in kabuki and Noh, has permitted herself.

As the drama unfolds, we are reminded of this Japanese tradition since the characters don't so much act as attitudinise, with slow hieratic movements, like figures on a frieze.

As a musical event, this would charm the birds off the trees. The timbre of the male singers - led by the baritone Peter Harvey and tenor Julian Podger - reflects classical polish, while the female singers, though pure-toned, favour a folky kind of belt.

What unites them, however, is a beguiling blend of conviction and joie de vivre, plus a uniquely deft mix of medieval musical sounds. Having taken the surviving manuscript's minimalism as their cue for harmonic inventions, the modal music that results creates a wonderfully dreamy ambiance.