Message #68664

[Nyki]

I love this story too.  To answer your questions:

The Doctor was trying to get to Metabilis III throughout the previous series, and he did (briefly) during The Green Death, when he brought back the blue crystal and gave it to Jo.  It was a very different place then, apparently uninhabited apart from monstrous creatures.

The Lama hadn't actually appeared before, but the Doctor had talked about his old mentor rather more fully than here in The Time Monster.

Without too many spoilers, Mike Yates left UNIT in Invasion of the Dinosaurs.