Well i am rather speechless after watching this episode i really can't believe they did this because they have just rewritten the back-story of Dr Who which is rather alarming considering the consequence of this situation from science fiction point of view yes this was fantastic episode but the fact they never show ripple effect of change the history really annoyed me.
I really hope they cover this in future episodes because changing his back story to where he never destroyed his home planet will certainly create a different dr who to current dr who.
A very cleaver use of 400 years calculation but it really took some liberties with the fact they use it 3 times for 3 time calculation was pushing it because no system could handle that much system information without crashing sonic screwdriver ?.
Sorry i understand this was for entertainment purposes but if you use common sense you can see problems with certain part of the storyline.
Overall it was very enjoyable episode to watch
