WTF. That's insane. I think out of every missing story, this is the one fans would want the most. I remember as a kid just reading the synopsis in my Doctor Who Program Guide book and being fixated on this episode.
It's a 12 part story, what's the point of only watching the first three parts?? Thing is, that they've done that indicates they have no interest in animating it as they have with other stories, which is a shame.
The Beeb's policy on animating seems to have morphed over the years. Initially they were happy to animate stories with a mix of existing and missing episodes, then apparently they decided they were only animated stories that were missing entirely. Who knows what it is now. Source: Some dudes at TrekBBS in the Doctor Who subforum.