thanks swebby!
It appears the Ancestry trees are completely wrong then. They have them born 1650 and in Sussex so no wonder I couldn't find them.
My money is still on Sarah born 1676 in Mayfield being our John's sister. I have a will somewhere for Henry Monk who left money to his daughter Ann Isted or Ested, can't remember without pulling out the will, married to William who I take to be William son of John and Ann Piper.
Kerry