Doctor Who's Fixed Points

Some day, hopefully when I'm very old, somebody may ask me if I have any regrets.  I'd do things differently.  I know I'm going to be a grumpy old man; but, there are things that I wouldn't change.  They'd be like the fixed points in history that Dr. Who can't change.  Like the eruption of Mr. Vesuvius, I need to make sure certain events happen, like Fleming.  I can stay in Quebec in 1989, but I have to go to Fleming in '94.  Like in Quantum Leap, there are things that should be fixed; but, they can't interfere with the fixed points in my history.