Graham Dilley was a cricketer, notable for playing for England in the 1981 Ashes series. He inadvertently created a divergent, apocalyptic timeline by his fielding in the third test. In the original, proper timeline, he made a spectacular catch which secured England's victory over Australia, and thus kept the Ashes at home. The new timeline was created when he dropped the catch consequent to a late distraction. This error incited a riot which led to greater social disorder, an assassination attempt on Elizabeth II and ultimately a wave of political deaths, anarchy, and war across the globe.
When the Fifth Doctor, Peri Brown and Erimem went to Leeds to see what had happened, they discovered that a younger version of the Doctor's fifth incarnation, Tegan Jovanka, and Turlough were already there and that Dilley's last-minute distraction was Tegan herself. The younger Doctor hopped back in time slightly to replay the events of the match. He, of course, couldn't directly intervene in the flow of events because he might have met himself and triggered the Blinovitch Limitation Effect, so he gave Peri and Erimem the task of stopping Tegan from affecting Dilley's play. At a moment when Tegan was separated from Turlough, they attempted small talk with Tegan, which annoyed a spectator. Soon Tegan was in a heated argument with the spectator and his wife, and Dilley noticed the brewing row. Tegan snapped at Dilley to keep his eyes in the game, which was enough to ensure that Dilley would catch the ball, give England victory and restore the timeline. (PROSE: Graham Dilley Saves the World)
In an alternate timeline where Polly Wright was manipulated into interfering with temporal nexus points, the time travelling Polly defended her younger self against the verbal attacks, and Dilley missed the catch. (PROSE: That Time I Nearly Destroyed the World Whilst Looking for a Dress)