"A Scene from 'The Amateur Gentleman,' 1936 showing Gilbert Davis (Prince Regent),
Athole Stewart (Marquess of Camberhurst??!!), and Elissa Landi (Lady Cleone)."
I did a little more digging on the Internet Movie Database,
doing a comprehensive search on "Farnol" (Incidentally,
they have his name spelt incorrectly as Jeffrey).
I turned up three(!) Amateur Gentleman movies - two silents in 1920 and 1926, and the one that Mark Blanchard noted from 1936. There was also a silent 1924 movie called Manhattan, based on The Definite Object.
Looking at the credits, the 1920 movie, which was made in England, seems to be true to the book, if we are to go by t he characters listed in the cast. All the familiar names from the book are there, although I am not familiar with the names of any of the actors (a bit before my time). Also Jeffery Farnol is listed as the only writing credit, so he may have had some control over the outcome.
The second, 1926, movie was made in the U.S. and sports at least one famous name in the cast - Richard Barthelmess as Barnabas. Again, the characters seem to be true to the book. Farnol gets only a story credit here, with presumably the adaptation done by Lillie Hayward. There is also a titles credit, for Tom Miranda, who is also listed as editor. (While it may appear that writing silent movie titles was a pretty simple job, it was actually very important, since it was the only dialogue that moviegoers would see)
The 1936 movie, made by United Artists in England, and co-produced by Douglas Fairbanks Jr, seems to bear little or no relation to the book. Coral Browne plays someone called Pauline darville, whoever she may be, while Margaret Lockwood plays Georgina Huntstanton and Irene Brown Lady Huntstanton. Esme Percy is Townsend and Athole Stewart is The Marquess of Comberhurst (?Duchess of Camberhurst?). None of these, as I remember, is a a character in JF's book. Several of these actors, including Marius Goring, whose part is not credited, were, or became quite famous in British films. Margaret Lockwood appeared in Hitchcock's "The Lady Vanishes" about the same time; Coral Browne, Esme Percy, Basil Sydney (Chichester), Hugh Williams (Ronald), Frank Pettingell (John Barty) and Gordon Harker (Natty Bell), all went on to become household names in the post-war British film industry.
Richard Addinsell was responsible for the music, and he became perhaps the premier film music director after the war.
Clemence Dane gets the top writing billing, so one can presume hers is the hand that introduced all these new characters.
The silent Manhattan, filmed in the U.S in 1924, and based on The Definite Object, has Edna May Oliver as Mrs Tropes (was she the landlady with one eye? and Richard Dix as Peter, so these were not inconsiderable talents for this Farnol-basede movie. Interestingly, the part of Joe Madden is credited to Gunboat Smith, who was a well-known professional fighter at the time of Jack Dempsey and Georges Carpentier, and a one-time champion. I wonder if JF, with his love of the boxing ring, was instrumental in this piece of casting?
The Definite Object, shot in the UK in 1920. I've never heard of any of the actors, but the name of the producer fascinates me: Countess Bubna. I see her as a White Russian emigre, formerly Eisenstadt's mistress, who fled Russia after the revolution. Well, it's a nice rumour, anyway.
I should think the likelihood of any of the silent movies still being in existence is rare; they were shot on nitrate stock, which just deteriorated in storage. There might be a chance that a print of the 1936 Fairbanks film exists still; is there perhaps a British Film Archives that we might contact?
All Farnol information welcome -