Having stirred the personnel pot earlier with the appointment of pianist Lars Vogt as music director, the Royal Northern Sinfonia--the United Kingdom's only full-time chamber orchestra stationed there at Sage Gateshead--is now challenging young U.K. composers to stir up its repertoire. Yes, the well-established tradition of prominent ensembles hosting rigorous competitions for emerging voices is summoned once more. This time, though, it's more of a mandate.