Hi, I have just come across this post as I am also researching the Grandy family from Dorset.
My line is through Thomas Grandy b1753 Sixpenny Handley, parents William Grandy and Grace Martin.
I believe William was born 1727 in Sixpenny Handley but haven't found a marriage yet. His father was possible William b about 1702 in Milbourne Port and his parents could be William Grandy b about 1640 in Longburton and Dorothy Rideout who were married 1660 in Lydlinch.
Williams parents could be James Grandy and Joyce King who were married 1623. He was made shefiff of Poole in 1656 and was buried in Longburton in 1689.
There appear to be earlier records I have come across for Robert Gronndy in 1592 and Robert Grndy in 1551 in Poole.
I havene.t been able to prove all the above facts but did have you any information that might confirm what I have found.
Kind Regards