The end credits contain the following: "...& I Paul McGann" The triple dots are as it follows Richard E. Grant's credit as Withnail (hence matching the movie title). While McGann's character's name is never referenced in the movie, either spoken or written, it has been identified as Marwood in materials issued in relation to the movie.
Special Production Consultant - Richard Starkey M.B.E. (Ringo Starr)