On their childhood travels Wolfgang and sister Nannerl dazzled aristocratic audiences with their double acts on the harpsichord. In the much-reproduced Mozart family portrait of c1780 we see them seated at the keyboard, Nannerl playing primo, Wolfgang’s right hand crossing over her left. Later, in Vienna, Mozart composed the first masterpieces for piano duet, represented here by the inventive set of variations on a chirpy, Papageno‑ish theme, K501.

I suspect that Alexandra Nepomnyashchaya and Richard Egarr are eyeing the two great Viennese duet sonatas, K497 and 521, for a follow-up programme. What we get here are the two cheery sonatas from the early 1770s interleaved with three duets by Johann Christian Bach, JS’s youngest son, who had befriended the boy Wolfgang in London. The only Italianised member of the Bach clan (in Milan he had converted to Catholicism, much to his family’s chagrin), Johann Christian was the master of euphonious galanterie, and probably the crucial influence on the young Mozart. His duets, each in two movements, are predictably polished and mellifluous, sometimes more than that – say, in the dark operatic dialogues in the finale of the C major, or the lyrical tenderness of the A major’s opening movement.

Mozart’s youthful sonatas, more or less contemporary with the Bach duets, mine a similar vein of sunny extroversion. Outer movements often sound like keyboard reductions of a symphony – a brass fanfare here, a thunderous tutti there. More consistently than JC Bach, Mozart, the born operatic animal, emphasises contrasts of register and dynamics. But with so many turns of phrase common to both composers, on a blind taste you could easily mistake one for the other.

Playing on a beautiful modern copy of a 1795 Anton Walter fortepiano, Nepomnyashchaya and Egarr are vivid advocates of music that is all about hedonistic enjoyment. The marked timbral contrasts between the fortepiano’s registers, duly relished here, enhance the music’s quasi-orchestral sound world, while the instrument’s undampened sonorities create a delicious haze of overtones. Tempos are lively but never harassed, phrasing buoyant and supple; and the players make telling use of the fortepiano’s sordino lever, most hauntingly in the ebbing conclusion of the Mozart variations.

Most crucially, Nepomnyashchaya and Egarr conjure a spirit of shared spontaneous delight, complete with ‘how about this?’ mini-cadenzas and spur-of-the-moment (so it sounds) embellishments. In his engaging note Richard Egarr suggests that these duets are ‘full of musical alleys and corners in which to run, hide and say “boo”’. That nicely sums it up.