ANAT: This may come as a shock to you, but you've just travelled two hundred years through time.
DOCTOR: Thank you. But I'm probably more familiar with the concept of time travel than you are. Now wait a minute. Before the time transference, I saw a Dalek.
BOAZ: You know of the Daleks?
1971 + 200 = 2171.
(Although none of the numbers used in the above equations are all that precise.)
I took this...
(The Doctor is exhausted, and kept on his feet by two Ogrons.)
DALEK: The Daleks have discovered the secret of time travel. We have invaded Earth again. We have changed the pattern of history.
DOCTOR: You won't succeed, you know.
DALEK: The Dalek empire will spread through all planets and all times. No one can withstand the power of the Daleks!
CONTROLLER: Take him away.
...To mean that they stopped the Doctor stopping them at the end of the serial in 1964.
They didn't reinvade later later on.
They altered history, reinvading at the same time, so that they never lost.
The Daleks could have knocked the Doctor off course so that he never arrived when he did, or they could have showed up with reinforcements an hour after the TARDIS left, or they could have always confidence tricked One into believing that their defeat in Dalek Invasion of the Earth was real when they had really just been play acting till he buggered off... Like at the end of the Sting, or the Sting II. The mark runs off assuming black is white and up is down and has no desire to confirm his or her facts.
Or they could have just killed him, Ian, Susan and Barbara.
Also in Audio land, Peri and 6 arrived in the England countryside last month, where they were talking to the weirdo's acting like it was mostly the 17th century all over again involved in a public stoning, when somebugger revealed to the Timelord the date: 2162.
"Let's go Peri, lets go now."
"What's the matter Doctor, shouldn't we help these people?"
"No we shouldn't. We shouldn't be here at all, and we really must be going."