Souska John
From Tardis Wiki, the free Doctor Who reference
Souska John played a Castrovalvan child who had a brief discussion with the newly-regenerated Fifth Doctor about basic mathematics. (DW: Castrovalva).
She is the niece of Caroline John, though this had nothing to do with her getting the part. According to Fiona Cumming, the director who cast her, the relationship was not uncovered until later in the production process.
