Nearly a quarter of a century has passed since Brigitte Fassbaender retired, quitting while she was ahead much in the way that another great mezzo, Janet Baker, had done some five years before her. The two singers, both supremely moving, could hardly be more different, but comparison in some ways helps highlight what makes Fassbaender, who turned 80 in July, who she is. If Baker is restrained, regal and noble, Fassbaender is urgent, impetuous and even mischievous. Baker’s voice was beautifully steady, Fassbaender’s in a state, it seems, of constant flux, a mix of rhubarb and cream – luxurious and rich but with a tart edge – with a fiery schnapps on the side. Add in a darting, questing mind (still very much in evidence when I met her five…