Either Jascha Horenstein (in 1954) or August Wenzinger (1950-53) are commonly cited as leading the first Brandenburg Concertos to be recorded at least partially on period instruments. In fact the Czech musical historian and pedagogue Josef Mertin beat them to it in this little-known set which has finally made it to CD.

Though the booklet pays detailed tributes to Mertin, aided by Nikolaus Harnoncourt’s widow Alice, the given date is no more specific than 1950, the location a Rococo Viennese hall, Casino Baumgarten, which more recently hosted the premiere of the Cinderella opera by the precocious Alma Deutscher. Microphones are placed some way back, you may assume for the sake of discretion in the First Brandenburg, which gets off to a rocky start. However, the merits of these remarkable performances gradually reveal themselves.

Those merits centre round the kind of humane, practical musicianship that has more recently distinguished Trevor Pinnock’s recordings. Pinnock leads from the keyboard as primus inter pares, whereas Mertin stood out in front of a group of mostly students. Among their number were Harnoncourt, Gustav Leonhardt (playing viola da gamba and not harpsichord, which was left to Bruno Seidlhofer) and Eduard Melkus on viola. All three were in their early twenties, marshalled by a steady but not inflexible pulse from Mertin and leader Edith Steinbauer, moonlighting from the first desk of the Vienna Symphony (which Harnoncourt would join two years later).

Using minimal vibrato, full tone and long bows, they contribute to a warmly responsive string body that makes the Third and Sixth concertos especially satisfying. The trumpet solo in the Second is again placed too far back to engage in full dialogue with the recorder, but it’s played with admirable polish on what sounds like a valveless instrument by Helmut Wobisch (later chief executive of Vienna’s ‘other’ orchestra) without rivalling the astonishing virtuosity of George Eskdale for Fritz Busch in 1935.

The comparison with Busch is otherwise germane; notwithstanding the scholarly background, Mertin was also far too good a musician to lapse into the kind of typewriter Bach that a one-dimensional idea of historical rectitude imparted to Baroque music in the middle decades of the last century (not excluding Horenstein). He allows Seidlhofer free rein in the huge cadenza of the Fifth (after an odd hiatus at 3'30" into the first movement) which is duly dispatched with terrific verve. Perhaps too few opportunities are taken for quiet playing in the outer movements, which can sound hard-pressed in terms not of tempos but of dynamic phrasing in a recording that favours the basso end of the continuo. However, the set is essential listening for Bach recording historians.