It wouldn't be Thunderbirds without the doddering Grandma Tracy providing comedy relief. And someone's got to help keep those Tracy boys from fighting amongst themselves. Sandra Dickinson will fill Christine Finn's shoes as the voice of Grandma Tracy. Dickinson is well known to sci-fi fans for a couple of reasons; she was the wife of former Doctor Who, Peter Davison, until their divorce in 1994. She also played Trillian in the BBC television adaptation of The Hitchhiker's Guide To The Galaxy and in later radio adaptations. Dickinson is renowned for her natural high-pitched squeaky voice, but can "do" a British accent. Hopefully, she'll be choosing the latter option for Grandma Tracy.