Plot: A special BBC Centenary edition of the general knowledge quiz in which four teams try to come up with the answers that no-one else could think of. Presented by Alexander Armstrong and co-host Richard Osman, with contestants Tony Blackburn, Scott Mills, Dr Maggie Aderin-Pocock, Prof Chris Lintott, Peter Purves, Lindsey Russell, Shanequa Paris, Mobeen Azhar and Baroness Floella Benjamin