Donald Garfield is a non-canonical character created for ITV's 2006 adaptation of the novel The Sittaford Mystery. He is a mysterious individual who checks into the The Three Crowns hotel early in the show using what is obviously a fake name "Smith-Jones". The character is portrayed by Matthew Kelly.
Much later in the episode, he reveals that his name is actually "Donald Garfield". He describes himself as an "officially-authorised Whitehall dirt digger". As Clive Trevelyan might be the next Prime Minister of Britain, he had been ordered to "sniff out any skeletons in his constituency cupboard." His mission was, however, rendered superfluous quite quickly after his arrival. Over the next days he showed an interest in Miss Elizabeth Percehouse and cannot understand why he keeps being rebuffed. This may have something to do with her backstory (see the article on her for details).
Jones is the name of a commercial traveller staying at the Three Crowns in the original novel. In the novel there is also a character named Ronald Garfield.