A couple familiar faces in this episode (which just aired in the US last night on SciFi):
Felicity Kendal played Lady Eddison, but I recognized her as both Rosemary from "Rosemary and Thyme" and Barbara Good from "Good Neighbors"
Christopher Benjamin played Colonel Hugh, but I originally know him as Mr. Henry Gordon Jago, the theater owner from the Tom Baker episode "Talons of Weng Chiang"
Also, I had to double check to be sure they weren't the same person, but Fenella Woolgar, who played Agatha Christie, bears a striking resemblance to Penelope Keith, of "To the Manor Born" and "Good Neighbors" fame.
I liked it. I figured being a Dr. Who episode there would be an alien twist somewhere. I never bother to try to determine who's guilty, which usually means I enjoy the reveal more.
I thought it worked fine as a stand alone episode but was glad it wasn't a two parter. My daughter correctly immediatley identified the alien as soon as the various characters were introduced. Loved the flashback sequences.