Book completely different
I enjoyed this film so much that I ordered the book online and read it last week on vacation.
The book is also good but very different from the film. Basically, the main character, whose name we never learn, comes to England on a boat (but not helped by a cute little kid). The rest of the book he spends in hiding, all alone, most of it in a tiny hole he digs for himself in a field.
There is no girlfriend, no rich brother, no scene in the German compound (just references to the main character's horrible torture). There is a Quive-Smith, and the climactic scene is similar, but that's about all. You should check it out; it's not very long, and it's well-written.