Two of a Kind kind kind

I haven't watched enough of Dr. Who or Dr. Whooves and assistant to truly critique your historical accuracy, but I know enough about both that I basically understand what's going on here (though I'm trying to decide whether you're pointing toward Ditzy and Dinky coming from the 'future' or only Ditzy coming back and finding herself as a filly [I would think a slight color shift in adulthood would be believable to those noting the difference in their hues]. Both Ditzy and Dinky's reactions work extraordinarily well for either situation). I don't know enough about the earlier Doctor to determine if his expression would have worked, but I know all the others' are spot-on.
In terms of your technique, you have a great eye for shadowing and depth. I'll be a little picky for a couple of things though. With the way you positioned the ponies' shadows, it appears that the sun is directly overhead (probably around noon-ish), so with a flat shadow like that, I would suggest giving their shadows a bit more details in certain areas. I.E. the lengthier tails should extend a narrower part, and Derpy's extended legs would probably give some small lumps out of the sides.
The shadowing and depth on all the buildings' overhangs is really good, but there appear to be no shadows at all on the ground. With the contrast of color you gave the ponies' shadows, it may have been worth giving the buildings' a little more attention. Even at midday like that all buildings always have at least a slight shadow in some direction. Also, if it is in fact around noon, the tops of the buildings seem to have improper shadowing. The only one that appears to have light shining directly on the top is the one in the back center with the blue timbers. The others (in fact all of them) have really great shadowing, but they would be best for a different time of day (either earlier or later, but not when the sun is directly overhead like that).
I want to repeat that I am being very nitpicky here. The shadowing is just something I tend to notice. That aside, this is a really great piece of work, from the ponies to the buildings to just about everything else. Just keep the whole 'time of day' thing in mind. If you're going to include shadowing like that, I would pick a time of day and stick with a particular angle of shadowing, like a 3pm shadow could angle everything to the right, left, or back depending on the characters' compass direction. Kudos on a job well done!