Hi,
Glad to see that this is the right family

. To assist you further - what exactly is it that you would like to know? I am curious whether you would like to go forward or backwards from Marmaduke (b 1795)?
If you are interested in finding out about your Graburn relatives in Canada - descendents of Marmaduke - then a moderator could move this to the Canada Board. There seems to be a lot of information available, and the Rootshchatters on that Board could help you.
If you have all the information you need about your Canadian relatives, and what you want to know is how your Graburns go back from Marmaduke and connect with Horatio Nelson's ancestors/descendents, then you would probably be better off on the England board, (or a military one)? Perhaps some Rootschatters have access to English parish records (or other records) that could help.
I saw your post on another site, where your relative mentioned the family tree showing the Nelson and Graburn connection.
http://boards.ancestry.com/thread.aspx?mv=flat&m=2&p=surnames.grayburnWere you able to get a copy? That cousin says the Nelson connection is a couple of generations before Horatio Nelson. But another poster on that thread thought that the connection was through a granddaughter of Admiral Nelson

.
There are some books by a Michael W. Graburn of New Zealand (I think). One is called "The Early Graburns".
http://www.rootschat.com/links/0hei/If you haven't contacted him yet, it might be a good idea to see what he knows of the family history.
RK