Noddy

Noddy was a character from Enid Blyton's books. When the Tenth Doctor was nonplussed at the extraordinary coincidence that Agatha Christie could be in the middle of a murder mystery or the idea of Charles Dickens seeing Christmas ghosts, Donna suggested the absurdity of meeting Blyton and finding her having tea with Noddy. She asked him if Noddy actually existed and he assured her that he didn't. (TV: The Unicorn and the Wasp)

Behind the scenes
Earlier in the same scene, the Tenth Doctor refers to PC Plod, an insult referencing a character from the Noddy series of books.

The Doctor Who? comic strip that appeared in DWM 190 had the First Doctor and Noddy (as BBC employees) discussing their shared hatred of changes they saw to Blyton's work in the BBC's New Adventures of Noddy show.

On screen, Noddy has been voiced most prolifically by Susan Sheridan, who played the character throughout the entire run of (1992-2000).