The final instalment of Donald Runnicles's examination of Tristan und Isolde and the music it inspired placed the third act of Wagner's masterpiece alongside the world premiere of William Mival's Tristan Still, a 20-minute orchestral piece that aims to summarise and subvert the impact of the original.

Mival's title is two-edged. Woven in postmodern fashion from scraps and shards of Wagner's score, it testifies that Tristan still haunts the western imagination like no other work and that its legacy is pervasive and inescapable. Mival, however, also aspires to stillness and calm, qualities that Tristan und Isolde conspicuously lacks. The opera derives its churning emotional force from unresolved harmonic and melodic suspensions. Mival breaks these elements down in order to refashion them into a ruminative meditation on musical stasis.

Slowly moving, penumbral chords chart the harmonic structure of the act two love duet without the chromatic tensions of its vocal lines and the erotic rhythmic throbs that accompany it. The sickly sweet violin phrases that Wagner uses at the opening of act three to convey the wounded Tristan's misery hover over the dense textures now robbed of pain.

The Wagnerian fragments ceaselessly spill into allusions to the later works they affected. Harp splashes and downward swooping string portamentos recall the Adagietto of Mahler's Fifth Symphony. The shepherd's cor anglais solo mutates into the opening of Ich bin der Welt abhanden gekommen from Mahler's Rückert Lieder. Pulsing brass chords, meanwhile, glance at the so-called Hauptrhythmus with which Berg closes each act of Lulu. The whole has a grave beauty, though its post-Romantic scoring and tonal harmonic palette might seem old-fashioned to some. Runnicles, hitherto ill at ease with the Tristan-inspired concert works, is happier here and conducts it with noble solemnity.

Throughout the series, Tristan itself has been magnificent. Runnicles allows the final act to unfold with death-haunted nobility, culminating in a performance of the Liebestod that is perfectly judged in its combination of the mystic and the erotic.

Christine Brewer's Isolde remains among the most sheerly beautiful examples of Wagner singing, while John Treleaven negotiates the gruelling challenge of Tristan's final ravings with tremendous vocal force. Hearing the opera over three, non-consecutive nights has been a major annoyance, though it has also been an outstanding achievement, no question.