This Southbank concert by GBSR Duo (the name derives from the players’ initials: percussionist George Barton, pianist Siwan Rhys) offered no Taylor Swift. Instead, they juxtaposed Aphex Twin and Karlheinz Stockhausen, a composer who subjected European classical music to shock treatment in the decades following World War Two. The two traded mild insults 30 years ago so the concert was billed like a prize fight, Stockhausen vs Aphex Twin. In fact, the two works performed were more complementary than antagonistic. Stockhausen and Aphex Twin: Together at Last, perhaps.