Hi Paul,
I suspect that James Dickson was the convict "James Dixon" who was tried at the 1838 Lent assizes at Antrim and sentenced to 10 years transportation for horse stealing. This James Dixon arrived in Sydney aboard the Clyde on 10 September, 1838. According to various records (tickets of leave, pardon etc) this James Dixon was a labourer, native of Antrim, born in 1817 - so this seems to fit with our "James Dickson".
I will send you an email with more information - I think you sent me an email last November but I haven't yet replied.
Cheers
Karen