Take a look at Michael Finnissy’s recent work list – it’s there, posted on his website – and ask yourself what’s going on. Could it be that the composer of Post Christian Survival Kit, Casual Nudity and Yob Cultcha has taken refuge in traditional forms at last, battle-wearied after decades of pressing uncomfortable and largely unwelcomed questions about music and its role in society? How else to explain an abrupt outbreak of generic titles like Piano Quartet in G minor, Organ Symphony and these Second and Third String Quartets, both written for the clearly loving-it Kreutzer Quartet.

The Second Quartet (2006-07) is a meditation on Haydn, written without a metrically aligned ‘master’ score. The idea, Finnissy says, is that individual parts should ‘drift slightly apart’, a strategy that messes with polite string quartet protocol. As Christopher Fox’s shrewd booklet-note makes clear, both works begin from the assumption that the string quartet is not only a form – it’s a medium too. But, say I, most composers forget that, and music that merely ‘works’ within terms set by romanticism, classicism or modernism – in fact any ‘ism’ – is of zero interest to Finnissy. He’s motivated by magnifying the string quartet from the inside, broadening the form by re-examining the medium.

And mapping the 45-minute structure of his Third Quartet (2007-09) over the architectural space occupied by Bruckner adagios elevates the basic building blocks into something grander and more voluminous. Finnissy’s 1986 String Trio has a similar relationship to Mahler; but this new piece takes the string quartet out of itself – and indeed beyond music – with a bold and wholly unexpected sleight-of-hand.

The first time he wedges a block of pre-recorded bird song inside his viscous counterpoint – each string line inching across a multitude of chromatic, rhythmic and sonic variables – it feels like a gestural jolt too far. But then the concept takes flight: the bird song episodes become more ornate and developed, as Finnissy destabilises the string quartet’s harmonic impetus by having them work off uncoordinated parts. Miraculously, it’s the idea of containing sound with a concert hall that begins to look questionable and manufactured, not the recorded inserts. So much for titles – only Finnissy could write what adds up to a symphonic movement, splice in bird song and label it ‘string quartet’.