Not only do both movies The Winslow Boy (1999) and An Ideal Husband (1999), feature Jeremy Northam as a character named "Sir Robert", his performances in those movies also won him the same two awards (Evening Standard British Film Award's "Best Actor" & ALFS Award's "British Actor of the Year").
At the beginning of the movie When Ronnie Winslow tears open the envelope and is about to remove the letter, the stamp and address are facing him, then in the reverse shot as he removes the letter the stamp and address are facing away from him.
Yes, the film and the play by Terence Rattigan are based on the case of George Archer-Shee.
Q
What book is this film based on?
A
Not a book but a 1946 play by Terence Rattigan, also called "The Winslow Boy", which is still available. You can read a detailed description at the Wikipedia: http://en.wikipedia.org/wiki/The_Winslow_Boy. The film script keeps close to the original text of the play, only moving a few parts of dialogue around, and adding a few sentences in the very last scene.