Hi Crumpet - my sis had grade 3 PP, and at her 36 wk scan her placenta had moved! It was only 2.5cms away from the cervix, so she was told she could choose whether or not to have the c/s.
She was going to go for a vaginal birth, but when she went into labour she had a bleed so ended up with a c/s anyway. But - there is hope for you that your placenta will move!
All the best