Hi Jo
Welcome to Rootschat.
The parish register for Dunstable has been extracted to the IGI up to 1876 and there is no baptism for a Thomas around 1838. As you know there was a Thomas age 13 in Dunstable in 1851, born London, probably Clerkenwell. However his father Joseph was born in Dunstable - christened 29 March 1807, son of John and Mary Medcraft.
www.familysearch.org if you're not familiar with the IGI. However that detail is premature until you can show that the Thomas who went to Australia is the one born 1838.
I've looked through Dunstable in 1841 but couldn't see them, not that I expected to as the family that was in Dunstable in 1851 that included Thomas 13 born London were producing children in Middlesex up to 1850. The same family, minus Thomas, was still in Dunstable in 1861 with the elder offspring giving a birthplace of Clerkenwell.
I would try posting on the London board for the 1841, where the family should be in St Lukes/Clerkenwell, Middlesex, Joseph and Louisa, plus children John George 10, Charles 9, Louisa 7, Henry 5;Thomas 3 and Ann 1. However that is not going to give you much more than you already know, apart from confirming siblings, and won't prove that he's the one who emigrated. The first 4 offspring are on the IGI, christened at St Andrew Holborn.
I can't see him on any later censuses, nor can I see a death after 1851 so it seems quite possible that he emigrated, either voluntarily or otherwise. Have you checked for any possible court proceedings which might have led to his transportation rather than emigration?
What details do you have of him in Australia - age at death, date of marriage and any details on his marriage entry/certificate, ship passenger manifest etc? Fortunately Medcraft is not a common name, which narrows your search for the right Thomas in England.
Hope this helps
BB
