I'm a direct descendant of his parents Walter James Derham and Jane Gulliver, through George's brother James. I, too, have been trying to find info about George for many years and have come against a brick wall. There are several George Derhams in Christchurch around the time, but apart from George's birth and baptism records and the 1841 census, I haven't been able to find him in any other censuses or marriage and death records. The other Georges I've researched are linked to different parents.
Most of the Derhams in Christchurch seemed to have been agricultural labourers or seamen. So the George, which I've seen as well in the merchant seamen records, might well be him. Apart from the error in the birthday, there's also the error that he's listed as Durham. However, there doesn't seem to be a George Durham born in Christchurch in 1828 and the Derham name has been recorded incorrectly as Durham before.