Although John Nettles affects a heavy French action initially, it virtually disappears as time goes on. John Nettles' character is French. When he is relating the story to the three men on the train he is speaking in English, but since he is meant to be a Frenchman speaking in another tongue, he speaks with a French accent. In the flashback scenes he is 'speaking French', and so does not speak with a French accent. So, his French accent does not disappear, it is used to denote the times when he is not speaking in his own language.