Pierre Boulez has been renewing his long acquaintance with the London Symphony Orchestra in two concerts at the Barbican. It is rather marvellous that the 83-year-old seems exactly the same on the podium as he always has, batonlessly gesticulating in his meticulous and ascetic way, and marvellous that his commitment to a repertoire founded on the classics of early-20th-century modernism remains so steadfast. Tonight, he conducts Bartok’s opera Duke Bluebeard’s Castle and gives the British premiere of Osiris, by Matthias Pintscher, a German inheritor of the modernist tradition (oxymoronic though the phrase is).

The earlier concert began with Bartok’s Concerto for Two Pianos and Percussion, went on to Schoenberg’s Five Orchestral Pieces - one of the century’s defining radical statements - and Stravinsky’s Chant du Rossignol, and finished with the five of Boulez’s early Notations piano pieces that he has so far managed to orchestrate.

As with many Boulez ventures, the programme seemed designed to enshrine his creative work within a canon of his own devising. Like both the Schoenberg and the Stravinsky, these pieces are studies in virtuoso orchestration, but even more self-consciously so. Where Schoenberg seems to be experimenting, almost chemically, with the expansive modern orchestra in order to discover new kinds of musical substance, and the volatile novelty of Stravinsky’s symphonic poem is grounded in his opera Le Rossignol, of which it is a partial paraphrase, Boulez’s glamorous and exotic orchestral fantasy is flagrantly self-justifying. Admittedly, there is the seed of the original 12 piano pieces, composed in 1945 when Boulez was studying 12-tone technique with René Leibowitz, but each of these is a mere 12 bars. The orchestral recensions, though brief enough, leave them far behind.

It is a clear case of the seed having to die before new life can flourish, but Boulez has always believed that you have to kill the past to make way for the future.

By these lights, the future is relentless massive tutti. Few large orchestral pieces so deftly engage all the instruments, all the time. Debussy, absent from the programme but perhaps Boulez’s main precursor, points in this direction, while the Bartok concerto was in direct contrast to such an approach. An orchestration of his Sonata for Two Pianos and Percussion, made for himself and his wife to play as penurious American exiles, the work wraps the brilliant and constructivist original in the airiest of instrumental gauzes. There is a fascination in the very parsimony of the scoring, in noting how little Bartok thinks it necessary to add. It is a far cry from the obsessive recomposition and “proliferation” of material associated not only with Boulez, but with Berio and other upholders of that modernist tradition.

Yet the concerto is tantalising none the less, because the original passages Bartok has not touched are charged with new meaning. They shine in a new, untampered-with way. The work isn’t often heard (though, oddly, it was given at the Barbican a few weeks ago by the BBC Symphony Orchestra concert under that Boulez protégé David Robertson), and Boulez’s account, with the pianists Pierre-Laurent Aimard and Tamara Stefan-ovich,and the LSO percussionists Neil Percy and Nigel Thomas, was sublimely persuasive.

Another of Boulez’s precursors, and his teacher before he went to Leibowitz, was Olivier Messiaen, a figure about whom he has always been ambivalent, just as many people are. One cannot deny the force of his genius, but what does one do with the insatiable religious ecstasy? The question was answered beautifully by the latest event (the first since February) of From the Canyons to the Stars, the Southbank Centre’s year-long Messiaen centenary festival. This was not a concert at all, but a sung eucharist for the Feast of the Ascension, held at Westminster Abbey and incorporating L’Ascen-sion, Messiaen’s four symphonic meditations that he recast as organ pieces (substituting for the third the sensational Transports de Joie), played at appropriate points (Transports as the exeunt music) by Robert Quinney.

Hearing Messiaen in liturgical context is like discovering the early-music movement’s approach to Bach. The encrustations of irrelevant usage fall away and the expressive essence stands nakedly revealed. Quinney was superb, the choir, under James O’Donnell, were captivating in Poulenc’s astringent Mass in G, and the whole high-Anglican ritual was serenely enacted, evening sunshine irradiating the stained glass.

Ecumenically, the festival moves to Westminster Cathedral this morning, where another Messiaen organ work, his Messe de la Pentecôte, is being integrated into the service – even as you are reading this.

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