Of all slippery figures in Line of Duty, the elusive “H” is the hardest nut to crack. “H” denotes the top dog in command of the organised crime; he or she known to be a senior officer, with direct links to corrupt cops.

Some people (*cough* Supt Hastings *cough*) believe this mystery ended with the apparent suicide death of Chief Constable Derek Hilton in series 4. Hilton had long been affiliated with organised crime, and his cover was eventually busted.

But Fleming and others have doubts that Hilton is the only, or even the right, undercover officer who operated in the balaclava gang.

Unsung talent: The ability to stir up some seriously dark office politics. This unknown player puts your average water cooler spat into the shade with his or her ability to spread suspicion and abject fear in the workplace.

Played by: Who only knows…

Images: BBC Pictures

Line of Duty series 5: The definitive guide to who’s who in AC-12