The Long Good Friday

In the late 1970s, Cockney crime boss Harold Shand, a gangster trying to become a legitimate property mogul, has big plans to get the American Mafia to bankroll his transformation of a derelict area of London into the possible venue for a future Olympic Games. However, a series of bombings targets his empire on the very weekend the Americans are in town. Shand is convinced there is a traitor in his organization, and sets out to eliminate the rat in typically ruthless fashion.
Who lit the fuse that tore Harold's world apart?