Throughout the last century, a certain London address was inundated with strange requests. People across the globe wrote to 221B Baker Street, seeking to procure the services of its famous fictional detective – that is, to persuade Sherlock Holmes to apply his deductive prowess to the most pressing conundrums of the day, from the Watergate scandal of the Nixon era to the mysterious and unsolved murder of Swedish prime minister Olof Palme in the mid-1980s.