Barrie Roberts' "Sherlock Holmes and the Devil's Grail", originally published in 1995. There is a fair amount of Arthurian matter therein, and the whole is an interesting read, if not quite up to Conan Doyle's standard. Holmes and Watson are depicted well, there is plenty of local 1895 color in the various British Isle locales, and there are some interesting characters for Holmes and Watson to interact with, plus an ancient, secret message for Holmes to decode.
   In the spring of 1895, an American inventor of an ingenious camera is the object of anonymous threats to leave Britain, escalating to the abduction of his son, Jay. Behind this criminal activity, Holmes detects the hand of Drew, his old enemy Professor Moriarty’s, lieutenant, an ex-Scotland Yard detective and worshipper of Demeter, the goddess of fertility. Our villain searches for a priceless treasure which, if possessed, would give him power over the whole world. The general problem with non- Conan Doyle novels is that the author creates a modern mystery with action to keep the reader's attention but fails to understand human character and motivations. Villains act contrary to their own motivations. Situations occur that in real life would just end in death for the hero. Still, it is an interesting book with Arthurian flavor. If you can find it in a second hand store at a reasonable price, buy.