This Edinburgh Quartet “Late Session” was the last of three which, in the words of leader Tristan Gurney, have been 'examining' the string quartet canon of their patron James MacMillan. Examination is an apt word for the venue - the Anatomy Lecture Theatre Edinburgh's 'Summerhall' complex - and the informal nature of these hour-long events allows time for pre-performance illustrated explanation of the works and post-performance Q & A.

MacMillan's 2011 For Sonny was written in memory of a friend's grandson, who died soon after birth. The short life and the resultant sorrow seemed encapsulated in the solo pizzicato theme for first violin, which ascends the first half of a minor scale before beginning a poignantly incomplete descent. MacMillan described this theme in a (now online) programme note as being “like a nursery rhyme”. This prompted me to wonder if I knew of any minor key nursery rhymes. Even those which obliquely reference disease, death and disaster seem, oddly, to be jaunty, major key affairs.

MacMillan's theme repeats unchangingly. However, repeatedly reclothed in varying harmonies, it assumes a chameleon character. The interpretation of these chords, whose bowed articulation contrasted effectively with the pizzicato theme, was impressive. The tone colour and intensity of playing matched variously the tenderness, spareness and worrying darkness of the harmonies.

These chords subsiding, the 'accompanists' then joined Gurney in a pizzicato arrangement of the theme, whose simple harmonies suggested a more playful Renaissance dance. In the lively acoustic of the steeply raked, wooden benched Anatomy Theatre, this passage emitted quiet vibrancy.

There then followed the first of three tremolando crescendos for the 'accompanists,' the dramatic nature of which could easily be believed to portray the volatile nature of grief. Melodic interest then passed to Mark Bailey's expressive cello. The pizzicato accompaniment of the upper instruments was replaced by light ricochet bowing, which the cello soon joined. Following a final statement of the theme, the piece ended on a simple unison. This five-minute work, which the Edinburgh Quartet premièred in 2012, seemed instantly accessible. For that we had this sensitively intelligent performance to thank, in addition to MacMillan's economic compositional acumen.

An institution of some fifth years, the Edinburgh Quartet has a considerable library, housed within which was MacMillan's 1982 quartet Etwas Zurückhaltend, composed while he was a student of John Casken at Durham University. In 2008 Macmillan revised it for the Edinburgh Quartet who premièred it in 2010.

As second violinist Gordon Bragg explained, the title is a mood and tempo direction lifted directly from Wagner's Götterdämmerung, meaning 'somewhat reticently' - a mood strangely rare in the work. MacMillan identifies with Wagner's world where mortals, their lives affected by intervention of the gods, are effectively 'living in combination with the numinous' and he sees a parallel with the mindset of a Christian composer such as himself.

Wagner also makes a thematic appearance in this quartet via quotation of leitmotifs, especially the 'Redemption through love' motif, which undergoes various treatments. The most drastic of these involves octave transpositions which replace the theme's sweeping, romantic nature with something altogether more vulnerable and searching.

In the Q & A, quartet members expressed their appreciation for the amount of freedom of expression granted them by MacMillan. This libertarian generosity takes quite a concrete form in this work in a passage whose pitches are specified, but without duration. As is often the case with improvised sections this resulted in a rhythmic freedom and urgency of articulation which it would be difficult both to notate and to rehearse.

More than Wagner, I was reminded of Bartók in the insect-like nature of some of the music - the buzzing sound being created as much by the narrow range of individual players' notes as by the close proximity to their neighbour's note rage. When the harmonies opened out, the sense of space was all the more enjoyable for the preceding claustrophobia. Macmillan also employs textural variety to great effect, employing harmonics, pizzicato, tremolando, solos, tutti passages and harmony which ranges from the highly dissonant, through the exotically tonal to the very simple. Indeed, the work ends with a solitary 'dripping tap' pizzicato on second violin, producing the dramatic combination of reduced activity with heightened tension.

Exiting each of the three concerts in this series, I have taken the chance to glance at scores left open on music stands, at which point my admiration for the playing experiences a sharp incline. Guest violist Fiona Winning seemed all the more impressive for being able to blend into such an established quartet in such a challenging piece. Happily, the work didn't prove too challenging for the audience amongst whose enthusiastic ranks were many young people - a sight to gladden the heart of those concerned for classical music's future.