Bohuslav Martinů's Piano Quartet no. 1 (1942) is a beautiful work. The first and third movements are sharp and witty pieces which touch on jazz influences almost as strongly as on Bohemian folk music. And the middle movement is a graceful lament, the outer sections written just for the string trio, with the piano only joining in for the central section, and then again for the soft, poignant final chords.

It was a pity last night that the Spitalfields Summer Music Festival audience were denied the ending of this movement, especially given the sensitivity of the Schubert Ensemble's interpretation. But it was nobody's fault that the tailpiece of Douglas Paterson's viola broke off the instrument during the performance, and after a longish gap in which an audience member called Nick raced home to find a replacement, the quartet launched straight into the finale, presumably because of time considerations.

The group were impressively unflapped by this pretty extreme interruption to the programme, and their rendition of the final movement of the Martinů was vigorous, fun, and in fact better balanced than the first movement had been, in which the cello line had a tendency to get lost in St. Leonard's Church's big acoustic. The professionalism of the Schubert Ensemble, now in their 29th year together, was apparent throughout what must have been a harder recital for them than most, and 'professionalism' in this case includes not just accuracy, but also deep musicality.

Huw Watkins' newly composed Piano Quartet, a Schubert Ensemble commission premièred in this programme, proved an unlikely substitute for the Martinů slow movement as well as an absorbing listen in its own right. Like the Martinů, it begins with just the strings, and works through a range of moods and styles while always retaining a hint of its gentle opening strains. Watkins is a pianist himself, and one with a penchant for chamber music – and I think this was evident in the piece's piano part, which mostly dealt in tasteful flourishes and ripples. I hesitate to use words like 'undulating' when writing of a Welsh composer's work, but there was at least a gentle sense of flow which underlay the whole ten-minute piece. What was most impressive was the compression of different moods into a single sense of character; the effect was a work of real personality, which will benefit from the second airing it will get next year, in the more balanced acoustic of Kings Place, as part of the Schubert Ensemble's 30th anniversary celebrations.

The concert concluded with Antonín Dvořák's Piano Quartet no. 2 in E flat major, a work recently recorded by the ensemble. The group's familiarity with the piece was abundantly clear, and their performance had a brilliant attention to nuances of phrasing and lyricism. The second, slow movement stood out most, with its beautiful cello melody richly played, and a wonderful sense of drift into the faster central section. It's a complex, subtle movement which shows that however overbearing the influence of Brahms might be, this fact does not make Dvořák any less excellent and talented a composer of chamber music.

The Schubert Ensemble are certainly superb advocates for his music, and their recording of this piece is bound to be exemplary. In fact, in concert it almost felt a little too polished at times, bearing the hallmarks of studio time and not quite creating the frisson of excitement of less inevitably perfect live performance. But as an account of the piece, this was extremely fine and always enjoyable, as were the renditions of the Martinů and Watkins works. The Schubert Ensemble are a great example of the benefits of spending time together making chamber music, with or without their first-choice viola.