I sent the following in response to Patrick's note, and intended it for the list but it went just to him, so I'm re-sending it to the list:
Doctor Who, like almost all long running stories is constantly being invented. Continuity and details are less important than the story.
One of my, now pretty well dormant, hobbies is scale modeling. I know people who are into, say, Star Trek modeling who strive to carefully catalog all of the little bits from various airplane, tank, railroad etc. kits that the modelers at LucasFilm used to build, say, the ultimate authentic replica of THE Millenium Falcon.
Problem is, there was no one original Millenium Falcon, there were many, it changed from movie to movie, and even within the same movie, because there were different ones built to different sizes, etc. including full-sized partial versions.
I have a friend who actually worked for ILM during episodes I and II of Star Wars, one of the last of their physical modelers (it's almost all CGI now). During his tenure, the ILM employees did a charity project, making things to be built on eBay for charity. Roy decided to make an accurate model of the AT-AT from the A.M.T. kit. As he was researching it, he was puzzled by something, and decided to ask one of the guys who was still there who had worked on episodes IV-VI. "I was wondering, how did the crew of the AT-AT get into the cockpit in the head, was there a hatch, or an elevator in one of the legs or ..."
The answer he got was, they just used green screen!
More to the point of Doctor Who. Here's an interesting excerpt of a panel with Steven Moffat at a San Diego ComicCon when he was asked about continuity. http://youtu.be/JRK4XSJXtQQ?t=6m30s
basically his answer was when you deal with parallel universes and changing time, it's impossible to have a continuity error.
And from earlier in the same panel:
The Time Lords are gone, the rules are probably off, since when did the Doctor obey any rules, hell go on forever...