Death of an ambassador

When the new ambassador from Esmeralda is killed in London, Tommy Hambledon becomes involved in the investigation. He goes to Paris, where Letord of the Sûreté is puzzled by some unauthorized help he's been getting. As an officer of the law, of course, Letord cannot countenance vigilante behavior, but does it hurt so much to get these jewel thieves off his books? Hambledon feels much the same, especially when the mysterious person saves his life...