Pleasantly enough, Britten Sinfonia went down a thoroughly unconventional route in celebrating their 20th birthday at the Barbican on Saturday, with a brilliantly varied range of new pieces mixing with chamber orchestra classics. With a stellar range of guests, they carried us along all the way from Purcell to Moondog, encapsulating the spirit of versatility and openness which makes the group what it is.

Mixing old and new was the order of the day right from the start, when Britten Sinfonia Voices’ delicate rendition of Purcell’s Hear my prayer dovetailed into Nico Muhly’s newly-commissioned setting of the rest of Psalm 102 for choir and orchestra. Entitled Looking Forward, this was standard Nico Muhly fare, with its range of slightly off-the-radar influences from choral music (Kenneth Leighton, Herbert Howells) blending with a lightly expressionistic orchestral texture. It made for an attractive opening number, though I can’t help feeling the text was a decidedly odd choice for a birthday tribute (“My days are like a shadow that declineth; and I am withered like grass”; happy birthday to you too).

No complaints, though, about James MacMillan’s One, a miniature also written as a birthday celebration. A single melodic line which travels elegantly around the chamber orchestra extracting a brilliant range of tonal colours, it was the perfect vehicle for such a deeply sensitive, musical ensemble as Britten Sinfonia, and also suited well the evening’s democratic ethos, which saw most pieces “directed” by an instrumentalist (here, first violinist Jacqueline Shave) rather than “conducted”. One’s closing sonority, the only chord in the piece, was a masterpiece of scoring in itself, and here it was clearly in the right hands.

The varied first half also contained some better-known pieces and numerous top soloists: Bach’s Double Violin Concerto with dream team Pekka Kuusisto and Alina Ibragimova, Britten’s Les Illuminations with Mark Padmore, and Prokofiev’s Classical Symphony. All were gorgeously performed, with a breeziness in the orchestral playing which suggested a band enjoying themselves – though the light playing style did lead to a less emotive slow movement in the Bach than might have been hoped for. But nonetheless, the two violinists played with a superb conviviality, Kuusisto in a very laid-back, contemporary style, Ibragimova with a slightly more incisive, Baroque tone but no less of a sense of fun. The Prokofiev was also wonderfully sprightly, though at times I wondered if the lack of a conductor led to a less adventurous structuring of the piece than could have been achieved. But that said, the performance’s dynamism and sense of line were a very fine defence of the approach taken.

As was no surprise, Mark Padmore’s Britten performance was stunning, with an incredible intensity to it. The whole thing felt at times more like a vastly magnified set of piano Lieder than an orchestral song cycle, such was the directness of Padmore’s delivery and the conciseness of the instrumental playing, here marshalled from the first violin’s desk by Pekka Kuusisto. Despite the sense of mystery this work draws from its text – prose-poems by Arthur Rimbaud – there is also a typically Brittenesque element of sweetness to it, and this was primarily quite a lush account of the score, with Kuusisto hopping up and down constantly from his chair, determined to draw out every richness from it.

Most of the second half was given over to Joanna MacGregor, firstly playing Bach’s Keyboard Concerto in F minor and then leading an augmented Britten Sinfonia through Sidewalk Dances, her arrangements of twelve pieces by Moondog. Moondog, if you’re not in the know, was a blind New York street musician who played his own music, often with instruments he invented himself, dressed as a Viking. His compositions, which are compositionally rigorous (often using canons) but also bright and poppy, deserve the serious attention they often receive, and MacGregor’s suite is a great way into his eccentric, joyous world. Picks included the melancholic “All is Loneliness & Voices of Spring” and “Bird’s Lament”, whose riff will be familiar to anyone who’s watched a TV advert in the last several years.

The Sinfonia were joined by Andy Sheppard on saxophones, Tom Herbert on bass, Kuljit Bhamra on tabla and Seb Rochford on drums for the Moondog set, which may even have been more talent than was strictly necessary (I’d have happily listened to Rochford and Bhamra on their own for the whole evening). But this was certainly a truly appropriate celebration of Britten Sinfonia, epitomising not only their brilliance in performance, but also their commitment to bizarre genre-twisting projects and experiments. This wasn’t just an anniversary celebration – it was also their first concert in the Barbican as their Associate Ensemble. Plenty more fascinating collaborations are surely guaranteed.