How much prune juice can i give my 8month old daughter ?

my daughter has been starting solids since she was 6months old and i been given her pleny of water and diluted juice and her diet is good i make sure she has plenty of fruit and veg and fibre. and she still constipated. my health vistor said to try prune juice . how much can i give her and should i dilute it. or should i just take her doctors x