There are two significantly different versions of this book. Eric Williams' original 1949 edition was written while military censorship was still in force, and is a simpler story, built around simpler characters.
In the 1979 edition, Williams sets the records straight on (particularly) one event that was invented to make a better ending for the 1949 book. The rest of the book is significantly revised and expanded, also, and the PoWs' language becomes less "comic-book" and more (one assumes) true to life.
Read the 1979 revision if you want the actual story - the 1949 is more like a Boys' Own adventure.