On paper, or described in programme notes, the Musical Offering, a set of variations on a rather awkward, angular theme by an old-fashioned elderly composer, sounds like a dry, academic exercise in contrapunctal wizardry – there are canons where one part is read backwards, or where the music is turned upside down and read with a different clef, canons where the players aren’t told where to start, and canons that never end.

The fact that the composer then presented the work to Frederick the Great inscribed with shamelessly sycophantic comments like “May the King’s happiness grow with the augmented notes, and with the rising modulation may the King’s fame increase” isn’t encouraging either. But what makes all the difference is that the score is also inscribed with the magic name “J.S. Bach”, and despite his rigid adherence to the rules he sets himself, the “Royal” theme flits gracefully from musical bough to another, dazzling the listener not just with its cleverness but with its lightness and beauty.

The piece has its origins in its two ricercar movements – a grandiose form that pre-dated the fugue, so old-fashioned even by Bach’s standards. The first, in three parts, was improvised by Bach on a visit to Frederick and when challenged by the monarch to make it six parts, Bach admitted that he’d have to go away and work on that. Harpsichordist Andrew Passmore began the Ricercar a 3 with a careful and delicate enunciation of the theme, fixing it firmly in our minds, then broadening into a very smooth legato as Bach begins his musical explorations. In the 6 part version, Bach adds a flute (Frederick’s own instrument) two violins and a cello. Violinist Jonathan Martindale gave his solo opening lots of space, letting the theme float gently through the air. As the other parts came in, the musicians kept their lines carefully separated whilst maintaining a strict unanimity. I don’t know whether it’s possible for a single keyboard player to perform this movement, but they made me imagine that that was what I hearing.

The five players from Royal Northern Sinfonia made the Musical Offering even lighter by their imaginative presentation. They performed in the round, and added their own affectionate wit to Bach’s music. The players briefly introduced each section with a brief explanation of how it was constructed, either in their own word’s or Bach’s and flautist Juliette Bausor began by reading from Bach’s dedication to Frederick, whilst Andrew Passmore idly picked out the theme on the harpsichord. Best of all was their presentation of the Canon cancrizans – the crab canon, for two violins. The music was written out on two sides of a long strip of paper, big enough for the audience to read. One player began at each end, and they walked along the paper, and round the other side – like the music itself, it may not sound good on paper, but it was a brilliant visual illustration that set us up nicely for getting a sense of how the other canons worked.

The obsequious quote I gave above was applied to canons 4 and 5, which used an increasing number of notes, and then a series of modulations. In strange opposition to Bach’s words though, the music takes a strangely mournful turn here, and violinists Bradley Creswick and Jonathan Martindale together sang out the beautiful line with a richly sonorous tone and some particularly graceful ornaments. These two canons were a soothing interlude before the most elevated construction, the Fuga canonica in epidiapente combining a canon and a fugue, which was given a grand, sweeping performance, allowing us to bask in Bach’s brilliant counterpoint.

Along with the canons and ricercares, Bach also included a flute sonata for Frederick, again based on the Royal theme and retaining canonic ideas throughout. Even more so than in the canons, the Royal Northern Sinfonia players abandoned academic aridity; Juliette Bausor and Bradley Creswick were hotly passionate in the opening Largo, and the Allegro movements fizzed with energy, with lots of fire coming from cellist Daniel Hammersley. There was a lovely dialogue between flute and violin in the Andante, against a beautifully lyrical cello line.

The extraordinary thing about the Musical Offering, is that Bach creates such inventive richness from unpromising material and this, above all, was what the musicians seemed determined to put across this evening. The order of the movements is not set in stone, and they chose, appropriately, to end with one of the two perpetual canons, music that could have gone on forever and I probably wouldn’t have minded if it had. Gentle touches of rubato allowed the music to float, suspended in time, and although it had to come to a close, the hall lights were gradually dimmed so that we could imagine that it was fading off into eternity.