Thomas Zehetmair/Ruth Kilius, Wigmore Hall

Bigger than the violin, the viola is tuned a fifth lower, with a darker, warmer sound, and with richer harmonics: while the violin flies high, the viola can connect us to the earth.

And when these two instruments are teamed together, the resulting sound-world has interesting possibilities. But they seldom are teamed: hence the interest of this concert by violinist Thomas Zehetmair and his long-time violist partner Ruth Kilius, who presented us with some remarkable fruits of this combination.

Since these were the two string instruments Mozart played, it was no surprise to find him figuring prominently, though the story of how he came to write his violin-viola duos is strange. Joseph Haydn’s brother Michael had been commissioned to write six such duos, but fell sick (he was a famous alcoholic) after writing four: when he was threatened with the sack, Mozart nobly filled the gap with the final two, which he allowed Haydn to present under his own name. Yet they are so typically Mozartian that nobody now could think they were by anyone else; Zehetmair and Kilius played them with lovely panache.

The other three works in this programme were rarities. Nikos Skalkottas (1904-1949) studied under Schoenberg and became the leading modernist composer of his native Greece. But his ‘Duo’ has a lyrical freshness and opens with some mellifluous dissonances which put one more in mind of Bartok; the way this pair delivered it made it feel like a very large work condensed into a very small space.

Bohuslav Martinu’s ‘Three Madrigals’ were composed as a gesture of defiance against the Romantic tradition, and to me at least were a revelation. The first was a dizzy two-part invention, the second came bathed in a suggestive haze of trills and tremolos. The third had the crazily impulsive momentum of a fantasia, with its moods changing like clouds scudding across the sky, until it achieved a luminous splendour. No other composer ever wrote like this: if it had echoes of Bach’s string partitas, these qualities were transmuted with a quintessentially Czech fancy.

Only in Heinz Holliger’s perverse and unfathomable ‘Drei Skizzen’ did this brilliant duo slightly overreach themselves: called upon to sing along with their instruments, they couldn’t quite hack it.