The sense of systemic racial injustice in policing that has fuelled the Black Lives Matter movement is shared far beyond the shores of the United States. In Britain, it is two decades since a top level inquiry into London's police force found it to be institutionally racist. How much has really changed? Stephen Sackur speaks to Leroy Logan, who was one of London's top black policemen until his retirement seven years ago. How easy is it to root out discrimination dressed in a police uniform?