Hi Julie,
I cannot help you directly with William's marriage, but I too am interested in the ABERCROMBIE family of Bray/Cookham/Maidenhead. I know that William was baptised at Bray in 1766, the son of George and Mary ABERCROMBIE. I am descended from Frederick ABERCROMBIE who I believe is another son of George and Mary, although I have not found a baptism to prove it. George and Mary had a daughter, Charlotte, baptised at Bray in 1769, but she died in 1772. I suspect that Frederick and several other children (Mary, George Thomas, John, Abraham) were born before George and Mary arrived in Bray.
Family legend has it that George was from a privileged family and he eloped with Mary. With a name like ABERCROMBIE, the family must originate in Scotland, but I don't know if George and Mary eloped from there. I wonder where their earlier children were born and what made them end up in Berkshire.
Does any of this ring any bells with you?
John
British Columbia, Canada