Peter Davison ( born Peter Moffett April 13 , 1951 ) is an English actor . He is best known for his roles as Tristan on All Creatures Great and Small , and as the Fifth Doctor on Doctor Who from 1981 to 1984 . Peter Davison ( born Peter R. Moffett ; on 13 April 1951 ) is a British actor , best known for his roles as Tristan Farnon in the television version of James Herriot 's All Creatures Great and Small and the fifth incarnation of the Doctor in Doctor Who , which he played from 1982 to 1984 .