Cultbox seems to have broken the story online, though they credit the Radio Times with the info: lots of names for the Christmas Special are out.
Sheila Reid is guest-starring as Clara's grandmother (you might know her from long-running ITV comedy Benidorm, but she played a deeply sarcastic housewife in "Vengeance on Varos"). James Buller is playing Clara's dad (he's not reprising the part from "The Rings of Akhaten", that was Michael Dixon.) Elizabeth Rider has a part as "Linda" (she was in SJA!), Tessa Peake-Jones is "Marta" (Only Fools and Horses, and other, stranger things, like the 1983 BBC production of "The Two Gentleman of Verona" and a version of "What the Butler Saw"). We have another Facejacker veteran, Kayvan Novak, as "Handles." Sonita Henry, who has a bit part in the reboot Star Trek movie, is Colonel Meme.
There's a few others: Mark Anthony Brighton (Doctors, The Bill, the usual suspects) is Colonel Albero (are we to infer there is going to be a slight military theme to this Christmas' special). Rob Jarvis (various things, including a multi-episode turn in Southcliffe) plays Abramel and Jack Hollington is Barnable. He looks like the obligatory child actor. It wouldn't be a Moffat Christmas without one, would it?
And the Adventur Calendar reminds us that "Orla Brady joins the cast as Tasha Lem". Off "Fringe", if that helps.