I would walk away too, i would not have a puppy until at least 8 weeks old and would not expect to pay for the 2 weeks extra keep. I agree the idea of the last 2 weeks or more is for socialising the pups and the biting part of it is very important especially in such a big powerful breed. We have seen so many threads on here lately about puppies biting and how do they stop them and some i would suspect are pups that are too young to leave their litter mates.
I would say £50 is little to lose in comparison to the time you hope to have your dog once you find the right one.
As someone said before would you feel confident that you could consult the breeder with confidence in the future if you had a problem. have they said that if you do have a problem or need to return the pup that they will gladly take it back.