There are three ways of dealing with time travel in Doctor Who (you might want to print this out for future use.)
1. Look, it's just a story, time travel is completely impossible, the whole thing's a farrago of lies, ooh, look at those monsters.
2. Time can only sometimes be rewritten, and the Doctor has a vast and terrible Time Brain that allows him to see when events can be altered and when they are fixed, but for us mere mortals, such insight would turn our brains to soup, ooh, look at those monsters.
3. Ooh, look at those monsters!
I like 3. It's quicker.