I can see one Joseph Peacock in 1851, born St Albans ca. 1830, a labourer.
I see Mary is down as 'married' in 1861. The closest match I see for Joseph in 1861 is:
J Peacock, convict in custody, mar, 28, Labourer, b. St Albans Herts
at Portsea Convict Establishment (RG9/637 148 42)
(I can't see any other likely candidate for this J Peacock on the 1851 census)