Hi Sue
They may not have even been in England in Census 1841 (but were definitely in Australia by July 1842).
The 1841 Census rounded ages down to nearest 5 years (or was supposed to). From what I can glean, we have no real idea of when they were born in the first place (and I'm considering Gile's age at death when I say this - ages on death certs are often wrong). Even if they were in England in 1841 and not with each other but at parents' houses, we have no idea who those parents might be or where. Also, the 1841 did not record marital status or relationships.
I'm also going to ask re: Hannah/Ann WELLS. Where did the 'Hannah' come from? Is the Hannah CARTER who died age 50 in Victoria 1868 proven to be Giles' wife? I cannot find a marriage registration for Giles marrying 2nd wife Ann SMITH in Victoria - did they marry there?
Finding them on a ship may be nigh on impossible - I've looked at some of the online lists for NSW and Victoria, and a lot of Mr Carter, Mr G Carter, Mrs Carter entries etc. No way to tell from that, unfortunately, more so if they travelled without children. The only thing you can narrow down at present is the time frame from Dec-ish 1840 to 15 Jul 1842.
I wonder if putting a topic on the AUSTRALIAN board here at RC might help fish out more about Giles and Ann? Lots of great chatters on there with access to all sorts of data and true local expertise. If you do that, a link to this topic for background info and to ensure no search double-ups would be needed.
To order & pay for the 1841 marriage certificate (can all be done online):
http://www.gro.gov.uk/gro/content/certificates/Use the GRO reference that was listed on FreeBMD for Giles, not for Ann!
Cheers
AMBLY