What's the name of the Doctor Who novel that featured a cave system shaped like a tree?

I've read it years ago, when I was a kid. I picked it up from the local library. It has the Fourth Doctor, K-9, and I believe Leela. It starts out with the Tardis reaching the end of the universe, and finding a colony on a newly formed planet. The colony is set up in a cave system that's shaped like a tree. The people didn't like Time Lords because ages ago the Time Lords ruled them like gods. The Doctor has to win their trust, and help them escape the colony for some reason. That's all that I remember.