Worzel Gummidge
Worzel Gummidge was a scarecrow that could walk and talk.
Worzel was a lookalike of the Third Doctor, specifically he had modelled himself on him because that incarnation of the Doctor was his favourite. He could be contracted for work as the Doctor through the Lordsalookalikes Agency. He could usually be found in fields scaring away Daleks and Navy larks. It was well-documented that he changed heads a lot at this time. (NOTVALID: The Doctor Who Almost Lookalikes)
At some point Worzel acquired friends in high places. He was once in the running for becoming the Sixth Doctor but the Fifth Doctor was disgusted at this idea and booted him out of the TARDIS without hesitation. (NOTVALID: Doctor Who? 65)
After the Third Doctor's two companions gave him their resignation letters, the Doctor decided to move on and combine his two greatest characters: Doctor Who and Worzel Gummidge. He called it Doctor Gummidge. He claimed he would fly around the space-time continuum scaring away birds. Brigadier Lethbridge-Stewart featured in the series, as did the Master who on one occasion exhausted the supply of barley on Gallifrey just by eating it. (NOTVALID: The Skivers)
Other references
Jon Pertwee was once abducted by a man claiming to be the President of the Worzel Gummidge fan club. (NOTVALID: Gemini Plan)
Behind the scenes
Worzel Gummidge being portrayed as a lookalike of the Third Doctor is, indeed, due to both characters being portrayed on screen by Jon Pertwee.
