Writer of Line of Duty has this message for fans after final episode The writer of Line of Duty, Jed Mercurio. Right, Vicky McClure as Kate Fleming and Martin Compston, who plays Steve Arnott The writer of Line of Duty, Jed Mercurio has thanked fans after the much-anticipated episode was watched by more than 12 million viewers. During Sunday night’s episode of the BBC One series, DSU Ian Buckells, played by Nigel Boyle, was unmasked as the mysterious H, the corrupt police officer at the top of a criminal conspiracy. Nelson-born Mercurio also addressed the series’ divisive ending, wr...