Grandma Tracy

In 2065, an elderly woman lived on Tracy Island.

On one occasion, she sat next to the swimming pool with Jeff, Brains, and Tin-Tin, as Brains conducted an experiment in the pool with his model submarine. The test was ruined when Alan jumped in the pool for a swim, disrupting the sonic beam. The old woman looked over when an alert sounded which indicated that John was calling in from Thunderbird 5 and remained seated while Jeff got up to answer it. (TV: The Man from MI.5)

Behind the scenes
While Grandma's appearance in The Man from MI.5 amounted only to a silent cameo meaning that she has no Doctor Who universe actor, she was voiced by Christine Finn in fifteen of the thirty-two episodes of Thunderbirds.