Thanks for the quick repsonse.
I calles SAA and they told me that out of London each fligth has the flat seats.
But if you compare the seats, you will see that the seat out of FRA is a kind of shell-shape. So the person behind you won't disturb you when coming out of his seat.
On the other side the flat seat on 744 out of London doesn't have the shell but has more legroom.