Thanks Hudsonhawk! This is just what I was looking for. When reading original BdUs war diarys I was thinking, what was the area C mentioned there. I thought it might have been SW off England and surprise surprise, there it was...
I hope I could find more information, I'll let you know if I can...
And for others: I think there were routes between neutral ports for neutral ships where U-boats didn't make any hostile actions. I have seen it mentioned somewhere.