The duo for violin and viola, as perfected in Mozart’s two examples, is a particularly satisfying form. The instruments are so close in timbre and weight of tone as to blend perfectly, yet each has its distinctive voice, suggesting a dialogue between two characters. The Bulgarian duo of Evgenia-Maria Popova and Dimitar Penkov have the confidence, vigour and virtuosity to present these dialogues convincingly; entirely so in the two more modern works. The Martinů comes across particularly well: the two players clearly relish its continual stream of manic inventiveness and give their performance a rhythmic buoyancy that compels the listener’s attention.

The elaborate double-stopping of the Spohr, often resulting in complex four-part harmony, presents no problems for these players – there are places where we feel we might be listening to a well-balanced, finely tuned string quartet. This performance is, however, somewhat spoilt by frequent fierce accents, at odds with the music’s generally elegant, decorative style. The effect is exaggerated by a close, airless recording. And in the Mozart, despite the high tonal and technical quality of the playing, I missed the subtle shaping of phrases that gives his music its natural life and expression. In the Andante of K424, for example, Popova places emphasis on the projection and fine quality of each individual note, rather than on the rise and fall of the melodic line. An admirably varied programme of music, then, but the performances give variable satisfaction. However, I can thoroughly recommend the Penderecki and the Martinů.