Paul Cornell, who's writing quite a bit of the good doctor these days, has a long essay on continuity -- in fictional universes in general and Doctor Who's in particular -- on his blog.

(I find it fascinating, but I also come at this from the comics side, where continuity is both a god that must be ritually appeased regularly and an anchor that's jetisoned or retconned out of existence every year or so. So I might think about this more than most SF readers.)