The BBC had this to say on her appearance:
"We’re delighted to reveal that Jemma Redgrave will be returning to Doctor Who for the show’s fiftieth anniversary special. She previously appeared in 2012’s The Power of Three playing Kate Stewart, daughter of the legendary Brigadier Lethbridge-Stewart."Jemma Redgrave will join Matt Smtih, David Tennant, Jenna-Louise Coleman, Billie Piper, John Hurt and Joanna Page in the special, which airs in November this year in the UK.
[Source: Doctor Who TV and BBC]