There's a series running over in Great Britain called 'Life On Mars', which is about a Manchester cop who finds himself thrust back in time to 1973. And his knowledge of the scientific methods used in the future don't always help him as he tries to adjust to life as a cop in the past.
In the episode shown this past week, the cop (Sam Tyler) got to meet one of his heroes in rock 'n' roll, Marc Bolan of T. Rex. As this happened about five years before Bolan's demise in a car accident, Sam Tyler risked screwing up the established timeline by giving Bolan advice that could have prevented his pre-ordained fate.
"Be careful driving," said Sam. "Especially in Minis."
Obviously Marc Bolan didn't heed the warning, because in Toobworld he still remains dead. How do we know? None of those chronovorous monsters from the 'Doctor Who' episode "Father's Day" showed up to devour the wound in Time.
Whatever Sam Tyler has been doing in the past, so far it hasn't caused any major upheaval in the established chronology of the world. Whatever outcomes do occur because of his interference, they must have already been established as occurring; all he did was to alter slightly the means to those ends.
We'll have to wait and see if he eventually does change History in a major way. And then when those chronovores ("reapers") don't show up, we'll just have to thunk up some kind of splainin to cover the gaffe.