Lady Ann Talbot, later Lady Cranleigh was an aristocratic young woman physically almost identical to the Fifth Doctor’s companion Nyssa. Unlike Nyssa, she had a mole on her left shoulder.
The Fifth Doctor and his companions Nyssa, Adric and Tegan Jovanka met Ann on 11 June 1925, at a cricket match and fancy dress party at Cranleigh Hall, in Oxfordshire. Formerly engaged to George Cranleigh, she became the fiancée of his younger brother, Lord Charles Cranleigh, after George was presumed dead during his hunt for the rare black orchid. Unbeknownst to Ann, George had not died but had been tortured, mutilated and driven violently insane by the Kojabe tribe.
At a fancy dress ball, Ann and Nyssa wore the same butterfly costume to play a prank the other guests. This nearly backfired when George escaped and took Nyssa hostage, mistaking her for Ann, but Charles and the Doctor were able to rescue Nyssa, only for George to fall to his death from the roof of Cranleigh Hall. (Black Orchid)
Searching for segments of the Key to Time, the Seventh Doctor’s companion Ace briefly visited Cranleigh Hall in 1925. She saw a woman who was either Nyssa or Ann Talbot. She could not tell which as she did not approach or speak to her. (Time & Time Again)
Ann married Charles Cranleigh the following year. The Doctor attended the reception and asked a favour of the new Lady Cranleigh: impersonate Nyssa as part of an elaborate plan to defeat the Osiran Nephthys. By having Ann accompany him to 1996 and disguise herself as an elderly Nyssa, The Doctor was able to trick Nephthys into travelling back and forth between 1926 and 1996 via her Osiran time corridor, culminating in her ageing to death. (The Sands of Time)