Twenty minutes is reckoned by psychologists to be the most that any of us can concentrate without the mind wandering, the legs becoming restless, the eyes gently closing, the head dropping slightly towards the chest. It’s also just about the time needed to serve a hall-full of people gin-and-tonics and tubs of ice cream, and to roll on the piano for the second-half concerto, ‘Heeeeeave-hooooo!’ The Proms are back on Radio Three for the summer season, and, with them, the nightly interval talks, rebranded in recent years with their own running title, Twenty Minutes, as if in celebration of the happenstance that necessity is in this case matched by perfect