Unlike the rest of of the West-Eastern Divan Orchestra's Proms thus far, this concert was not a sandwich. It was more of a chocolate cupcake, with a creamy topping of Beethoven and a strong, substantial body made of Boulez. Not one for the dieters, but a wonderful late-night snack before the next day's roast dinner of Beethoven 9.

Pierre Boulez's Le marteau sans maître ("The Hammer Without a Master") is, as was rightly said before the performance, a real staple of 20th-century classical music, probably the key work of the 1950s. Despite subsequent imitations, innovations and changes in fashion, it remains a lastingly contemporary piece, as fresh today as ever. And of course, it remains as open to interpretation – however specific Boulez's score – as any great work. In fact, as an insightful Ivan Hewett programme note observed, what's most amazing in the work is the paradoxical coexistence of an atmosphere of freedom or lightness with the obvious rigour of the composition. A bonbon made by Heston Blumenthal, perhaps.

For six instruments and alto soloist, the work contains nine movements, including four vocal settings of three short works by the Surrealist poet René Char. These are placed in between various instrumental movements which are preludes, postludes or "commentaries" on the poems. The final movement, " 'Bel édifice et les pressentiments', double", is the second setting of that particular poem, and in it the alto gradually becomes subsumed into the instrumental texture, moving from singing to humming to silence (and in this performance sitting down). Her place is taken over by the alto flautist (standing up), who concludes the work with a perplexed duet with percussion. It's as confusing and unwieldy a composition as its intricate structure (and indeed its title) suggests, but it's also schematic, enthralling, and – most importantly – it sounds completely beautiful.

Six soloists from the West-Eastern Divan Orchestra performed this piece last night with François-Xavier Roth conducting (a substitute for Boulez himself, who had to withdraw), and it sounded delicious indeed. There was a warm edge to the tone which spoke of a group as immersed in Beethoven as Boulez – as indeed they are – and the technical playing was impeccable. Roth conducted with great control, letting the work swing in all the right places (the two commentaries on "Bourreaux de solitude" have an amazing sense of drive to them) and allowing the eclectic sonic glaze of the work to shine.

Alto Hilary Summers has copious experience with the piece, and she correctly took the role of a real ensemble member, rather than a soloist in the more traditional sense. It's a low-lying part, at times at least, and her tone blended beautifully with the alto flute and the viola, delivering the bizarre poetry syllable by syllable with the requisite detachment, and dying away perfectly at the close.

While the performance was top quality, however, I think this might have been the concert where the Boulez/Beethoven pairing – which seems to have worked pretty well in the orchestral concerts – ate itself. Le marteau was prefaced by a rather slight, early Beethoven piano and wind quintet, Op. 16, which didn't seem fully justified. In a brief pre-concert talk, Tom Service gamely attempted to draw some connections, and it is at least true that both works date from earlier in the careers of their respective composers than those works heard in the orchestral concerts. But while Le marteau is a real landmark composition and the piece that made Boulez, the Beethoven is not a major work; in fact, it's more or less a homage to Mozart, in the same key (E flat) as Mozart's own work for the same forces and never really threatening to cause much of a stir. The players performed it very neatly and in a Romantic manner that will not surprise anyone who's already heard the full orchestra under Barenboim, but the whole performance had the nature of an amuse-bouche to it – which seems an odd way to treat Beethoven, especially when the concert is a supplement to a symphony cycle. Still, Le marteau more than merits such attention, and Beethoven will be a pretty meaty main course tonight.