Hope this is also of interest.

If this is your John Basset, it seems his parents didn't marry. An Elizabeth Chudley died in 1842.

Deaths Dec 1842 CHUDLEY Elizabeth Kingsbridge 9 299

If Elizabeth was unwell John may have been cared for by relatives which could explain not finding him on the census with his parents.

http://boundforsouthaustralia.net.au/journey-content/dartmou...
gives info on emigration from Dartmouth to Australia.
Good luck.