Since you caught my error (US Government works), I feel it only fair to catch yours.
The 50 year rule was changed in the 1988 copyright act. From 1 August 1989, unpublished works only have copyright for lifetime+70 years. But there is a fifty year transition period. For unpublished works by authors who died before 1969, the copyright expires at the end of 2039. There's going to be a lot of stuff, published and unpublished, coming out of copyright in the UK on 1st Jan 2040. (Including your hypothetical letter by Charles Dickens.)
Originally Posted by HarryT
To answer the original poster's question, in the UK at least, a posthumous work has copyright protection for 50 years from the end of the year of its initial publication. The date of the author's death is not relevent in this circumstance. Eg, if I were to discover a previously-unknown letter written by, say, Charles Dickens, and publish it today, it would enter the public domain on 1st Jan 2060.