Still no luck with Benjamin in 1861 although there is a Rebecca Lake working as a servant in Penzance who was born 1836 in Chelsea. Which could explain how they met.
In the 1841 census Joseph father of Benjamin was described as "Junior" suggesting that his father might be Joseph also.
I have found this family in Penzance who could be Joseph's parents and siblings. The occupations certainly fit!
Back of St Clare Street.
Joseph 65 Labourer
May 64
Willm 25 Mason
Richd 20 Mason
May 30
All born in Cornwall.
Ref HO107143/ 2 f 29
Also, if you look on family search you will find a baptism of Joseph Carbis in Gulval son of Joseph and Mary on 30 Oct 1807.
There are several Carbis baptisms in Gulval to a Joseph and Mary including a William in 1812 , Richard in 1820 and a Mary in 1806 - these fit with the family in 1841 (remember that the ages of those over 15 were rounded down to the nearest 5.
Take care when looking at the family search site as I note there are submitted and extracted entries. the extracted ones are the safest to use although any of particular importance should be checked against the parish registers.
Well, enough from me now. When you have digested all this do ask any questions.
Barbara
Edit
Have just seen a marriage for Joseph Carbis of Gulval to Mary Bowden at Ludgvan 29 Aug 1801. These would be Benjamin's grandparents.