You may be looking for Tom Broady.

Tom Braudy was an astronaut. He was forced to remember himself as a ten year old who liked playing with Lego. He wanted his grandmother Mrs Braudy to get Sky. He met the Eighth Doctor, Charley Pollard and C'rizz when they bought an ice cream. He was trapped on a planet after a crash, and was regressed to a child. When Charley and the Doctor took him aboard his spaceship he remembered his real self, then ran away. The Doctor rescued him in the TARDIS, where he read a book. (AUDIO: Memory Lane)

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