The comedian Lenny Henry in a new book has backed calls for the UK to pay more than £18 trillion in compensation for its role in the transatlantic slave trade. It's a figure that some people think is absurdly high. It comes from a report written by an economic consultancy called The Brattle Group. They tried to quantify the impact of slavery on black people in North and South America, and the Caribbean. Adam Fleming speaks to one of the co-authors, Mary Olsen.