just a point, the same doctor (ie david tennant incarnation who visited qv and caused torchwood) can't travel to parallel worlds becasue he only came into being after the time war and that was after the doorways to parallel worlds werfe closed by the tome lords
just re-watched the second last eps of the 2005 series (i love it, only time i watch big brother!). the answer to one of the questions in the weakest link game was TORCHWOOD. so, another paradox. the past was already changed to allow that qv has met the doctor, therefore the doctor (DT) must have already been there, therefore we cannot change the past as everything is fixed? then obviously there is no point to life as it is just a series of preplanned events that have to repeat each other to infinity, including my writing this post and deciding to lie down and not bother any more....
don't like the sound of that one!
i've always been more of a fan of freewill and being able to change things. the lateral time theory that is seen so often in comics and graphic novels (just going through x-men again
)