Hi Nicholas, I think we may have a connection from your grandmothers side.
My grandfather is named John Evelyn De Haan Pereira b.1926. His father was named Barnard De Haan Pereira b. circa 1900. His father wasR.L. Pereira who apparently drafted the constitution of Sri Lanka. My grandfathers great(?) uncle was named Walter....... Do you have any more information regarding Johanna De Haan Pereira? eg.was she Javanese? did she have a dutch father or grandfather?
hope to hear from you soon......i will be getting in contact with my grandfather soon so he may also know about this side of the family.