Well - I bought the marriage certificate.
It states that Henry Allison was born in London, circa 1835 to Thomas Allison and Mary Davis/Davies/Davids (hard to make out).
Thomas is listed as an upholsterer, Henry an undertaker.
I found something about Thomas, and who I believe is another son of his called John, here:
http://www.monkhouse.com.au/auhistorySo it seems Thomas might have been born in Liverpool in 1806, and moved to Camberwell in London as an undertaker, cabinet-maker and blind-maker. This explains why Henry would list his birth in London too.
I think Henry came to Melbourne, Australia in 1854, on the Bulgersteyn. Aged 19 and on his own.
It may be his brother John, born 1944, followed Henry out here in 1862, on the Sussex, aged 18. Again, on his own.
I have found the families of these men in the Victorian BDMs - which is great. I even found Henry's will online.
Meanwhile, I think Thomas died in Camberwell in 1884.
However, I cant find any trace of Thomas, Henry or John in the UK - nor their mother Mary Davis - from the early years of the family. So I have no documentary evidence that the Thomas and John found at the link are related to Henry. And I can't find any trace of Henry in the UK. Its very strange - does anyone know of anything I havn't tried?