From its origins in Mesopotamia in 3,000 BC, the history of Britain’s favourite tea-time snack takes many surprising turns - from biscuit recipes made by alchemists, to sponge fingers dipped in wine by aristocrats. Author and historian Guy Walters talks to biscuit guru Elizabeth Collingham about the history of a treat which Britain eats more of than any country on Earth (and how eating Custard Creams was once seen as daringly cutting-edge).