Just a word of caution - I think you need to careful in assuming the "siblings" you mention are actually those of your James GIBLETT. As far as I can see the only documents you have for his origins are his marriage cert in East London, and possibly the 1841 census which also has "born in county" ie Middlesex (though this is not always correct) and the passenger list which says Bristol.
Whilst it certainly looks possible that Bartlee Stroud GIBLETT is a brother (having the same father's name and occupation), and the passenger list cited by wivenhoe has James' native place as Bristol - again it's not proof.
Will keep looking, interesting family. 
Yes I agree - not genealogical standard proof by any means but sometimes one has to take a leap of faith to get anywhere, particularly when there is a paucity of documentation - not publishing anything yet - just working through the various possibilities. However, there is a bit more than the census and the marriage certificate:
The Slains Castle passenger manifest states he is from Bristol which is interesting because if the 1841 census is right he spent 10 years in London before leaving for Australia. Did they ask for a birthplace? I'm not sure but other documents created while he was alive have his birthplace as Wells, Somersetshire or just Somersetshire. These documents include Eliza Ann's birth certificate, Eliza Emma or Mary Emma's birth certificate and Mary Ellen's birth certificate. It is possible of course that he was born in Wells but grew up in Bristol and if we follow the life of the William Giblett, currier (purported father) he was resident in Bristol at the time of his marriage but married a woman from Huntspill, Somerset and was married there. Apart from James, I have 12 other possible children of William and Ann - the ones that had a confirmed father, William Giblett, currier were all baptised in St Cuthbert's, Wells. 2 married in Temple, Bristol and 1 married in Huntspill, Somerset so there seems to be some mobility even though William Giblett, currier maintained a business in Bristol and died there in 1829. Hope this makes sense! It's a bit like searching for a needle in a haystack....
I did have another thought regarding the 1841 census - if his age is correct (19 -it doesn't appear to have been rounded up), then he was born in 1822 and not 1820 as other documents such as his death certificate suggest. This would fit with the fact that William and Ann had twins in 1820 who lived for 10 days. They also had another child, Eliza, born 1823 and not baptised until 1836. Perhaps they just never got around to baptising James?
