Up front, I must say "I am not a lawyer." Heed the advice given above and consult a lawyer specializing in copyright law.
That said, it seems clear to me right now that publishing in the UK should be fine, but you could open yourself to a legal challenge from the Conan Doyle estate if you publish your work in the United States and do not contact the estate and purchase a license for your work.
The reason I say this is because the recent films from Warner Brothers, and the series from the BBC (plus others), received a license from the Doyle estate. So did the series "The Young Sherlock Holmes" by Andrew Lane, which specifically notes on their website that "The character of Sherlock Holmes is in the public domain in the UK, and is thus free for use. The character is still in copyright in the USA, and will remain so until around 2020. Any book published in the USA which uses Sherlock Holmes as a character must be authorized by the Conan Doyle Estate."
That said, there is a law suit currently in progress against the estate in the United States asking for a declaratory judgement that copyright has expired on the works of Sherlock Holmes. According to recent press reports, the estate did not respond, and the judge has entered a default against the Doyle estate. Still a few legal hoops to jump through, and the judge must pass their ruling.