The granny lived in the 21st century. She was driving on a scooter.
When Sarah Jane Smith, Clyde Langer, Rani Chandra and Luke Smith were chasing a Raxacoricofallapatorian, they stopped at the end of a street. There were tree people standing there, the granny, a girl and a postman. One of them could be the alien. Rani thought it was the granny. She believed that her skin didn't fit properly. It wasn't the granny, she had vinegar in her basket, which was fatal to the Raxacoricofallapatorians, who wouldn't get that close to it. (WC: Find the Alien)