Hello Robin,
If you read from the top of this thread you will have seen that Greg's Robert Corkill turned out to be bapt Maughold 29 Aug 1819 son of Robert Corkill & Esther Looney, not the other Robert Corkill mentioned, who was bapt Maughold 11 June 1820, son of Peter Corkill & Elinor Clague.
I'm assuming that you have Robert's Australian death cert which could have given you his age and his parents' names, as the reason for thinking that he was this Robert Corkill? Do you have your gggrandmother Mary Ann Corkell's marriage and death certs, and do these give parents' details? (How much info. given depends on which state holds the records, but it is always worth getting these before trying to find overseas records - and can save a lot of expense if you are on the wrong track.)
Can you please give more details about Mary Ann Corkell, and what records you have which make you think that she and Robert were siblings?
Sue