To be followed by concerts on the 24th and 26th, Prom 9 on July 23rd starts one of most unusual cycles of the 2015 BBC Proms: the conclusive stage of the Beethoven marathon started four years ago by Leif Ove Andsnes and the Mahler Chamber Orchestra. On the bill for this first panel of the triptych: Beethoven's first and fourth piano concertos, as well as Stravinsky's astonishing ballet Apollon Musagète. A superb musical adventure which leaves us impatient for its next phase!

For four years, Norwegian pianist Leif Ove Andsnes and the Mahler Chamber Orchestra have been criss-crossing the world to perform their Beethoven programme in its major concert halls. It hardly needs saying that by now, Andsnes knows every note of the master's five piano concertos by heart – indeed, by fingertip. As well as playing, he conducts, from a piano facing the orchestra with its cover removed. For this first night of the BBC Proms marathon, the last section of this trek, he begins at the beginning with the Piano Concerto no. 1 in C major. From the first notes, it's obvious that the performers are in their comfort zone: their playing is impressively natural and, more than anything, radiates pleasure, musicality and passion. That's as true for the orchestral players as for the soloist: the technical challenges were overcome long ago, beginnings have been worked through hundreds of times, the interpretation has been honed as the concerts have proceeded: by the time we've reached Prom 9 this evening, the result is close to perfection. We can enjoy the balance between piano and orchestra, which sounds like the obvious, the impeccable honogeneity in the nuanced inflections, the precision of each sound (depth, rhythm, colour). Andsnes displays a particularly fresh sort of impishness, which magnificently renders the concerto's energy and sheer joy. If the piano has a clear timbre which is never muddied with the ensembe, it's because a true chamber music feel is present between all the musicians, in the sense that they are all listening to each other an their attention to sounding properly in ensemble never falters.

Some brief alterations to the stage setup allows the strings to return on their own in an unusual configuration: everyone standing up except for the five cellos, seated in a circle in the middle. Stravinsky's ballet Apollon musagète is written for strings alone, this being one of the main things that imparts its unusual character. But this music, with its broken up structure of two tableaus with variations, astonishes is every way, mainly disconcerting us with its constantly moving harmonies. These are overlaid upon each other as the work progresses, they start up or fade away in unexpected ways, they make one shiver at every moment. Caught up in this atmosphere, the term “neo-classical” (which conjures up an overly rigid stylisation) doesn't immediately spring to mind; rather, one thinks of the work as surrealist or even mystical. The score is played marvellously, the strings constantly in dialogue with each other, with the slightest rhythmic change precisely in place, which allows the work's soul to shine through. It's a fascinating moment with a beauty that's unique, bewitching, ineffable.

After this fantastic intermezzo, the piano and winds return to the stage for Beethoven's Piano Concerto no. 4 in G major. The first movement confirms what we had already understood from the first concerto: everything is so perfectly executed that it's hard to think of a single detail that one would want to change: the work is interpreted exactly the way we want to hear it. The accents and rests are worked in such a way as to bring to the foreground the various melodies or rhythmic lines that form the score's backbone. Everything is played with transparent harmony, with perfect coherence and a total absence of unnecessary effects. The pianist is in charge of the tempo: he messes around with it slightly, but always in the knowledge of what he's doing and always in conjunction with the orchestra. The second movement is simply overwhelming. It's a pure, unadorned song, whose breath is immortal, whose truth is deep. As the audience holds its breath, disarmed by the poignancy of the emotions, the musicians launch into the third movement , and we are once more carried away in a whirlwind of gaiety thanks to the piano which leads us in a dance which boils over with life and unquenchable excitement. The encores (a pair of Beethoven bagatelles)  weren't enough, because we are merely left with impatience to hear the other concertos, and to allow ourselves to marvel at the inexhaustible inspiration of Leif Ove Andsnes et du Mahler Chamber Orchestra.