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The Daily Record are claiming that Lindsay Duncan is to the the Doctors assistant for the currently unnamed episode to follow Planet of the Dead!
Lindsay Duncan, who is starring as Baroness Thatcher in the BBC2 drama Margaret, will play Adelaide, the Doctors cleverest and most strong-minded companion yet.
Duncan said I'm thrilled to be involved in Doctor Who. I've never done anything like this before and I'm really looking forward to working with David Tennant and the Doctor Who team.
The second 2009 episode is written by executive producer Russell T Davies and Phil Ford and will be screened on BBC1 later this year.
Note that I can't find any other reports about this, even the BBC press office have nothing about it, so there is a chance it could all be made up, but it does seem very possible!
I expect a post-Easter conformation of the next episode title, names of some stars perhaps (companion included) and possibly some news about Matt Smith, his costume maybe?)