The name "Withnail" came from "Jonathan Withnall" who was an upper-class, friend of Robinson's father. The character of Withnail however, was based largely on Vivian MacKerrell - a friend of writer and director Bruce Robinson who he met at drama school in 1964.
A biography of Vivian is available from Quartet Books and written by Colin Bacon. This memoir encompasses the half century from the end of the 2nd World War to the height of the ecstasy era.
In the introduction to the Withnail and I screenplay, Robinson describes MacKerrell as a hedonistic, likable, hard-drinking guy. He was one of the people who lived in the Camden house bought by David Dundas, their drama school friend later who worked as music supervisor on the Withnail and I soundtrack.
From a wealthy "shabby genteel" background, Robinson later described some of Vivian MacKerrell's traits: acidity, pompous cowardice and a very pungent sense of humour.
Although no lines in Withnail and I come directly from the mouth of Vivian MacKerrell, one of Robinson's diary entries mentions the near-legendary quote: "If there's a God, why are arses at the perfect height for kicking?"