Hmm - looks like you have gone wrong right from line one of your post!!!
The marriage at St Ives on 5th August 1787 was actually the marriage of
JAMES Berriman and MARY MARTIN!!
Their son
Thomas was baptised at Towednack 27th April 1806 and then he married another (and much younger)
Mary Martin at Towednack 6th August 1825
Now to see if we can deal with the problem of James Berriman in the 1851 Census.
Firstly - the James who married Christian Sampson was the son of Mark and Elizabeth Berriman and baptised at Towednack 13th May 1759. He was buried at Towednack 9th June 1819 age 62.
There appear to be only a couple of possibilities that might answer the question of James Berriman in the 1851 Census. The first of those was that rather than James Berriman this might have been James Martins the father of Thomas Berriman's wife. Problem with that is that I have James Martin baptised at Towednack in 1774 although I see now that I have another problem with this Martin family as there was also a James baptised in 1770 to the same family (Richard Martin and Grace Uren). As there appears to be no burial this 'could' be a case of a private baptism in 1770 followed 4 years later by a public baptism. That is an issue for me to try and sort out another day when I (hopefully) get a chance to have another look at the Martin families of Towednack and Ludgvan.
I also checked the 1841 Census and there appears to be no trace of a James Martin born around the right time to have been Mary's father so that would seem to rule out that option.
The second option may be the better one although it is clouded by the fact that in the 1851 Census James Berriman is recorded as 'father' of the head of household. However, we all know by now that information in the Census is not always to be relied upon!!!
So, let's think outside the box a little and imagine that the man involved truly is a James Berriman. We know he cannot be the father of Thomas because he was buried in 1819. But even if we imagined this was the father of Thomas and that he was buried in 1852 at the age of 83 and, therefore, born about 1768 or 1769 then he would have been only 18 at marriage. Perfectly legal of course but a little unusual.
Also more unusual given the possibility Christian Sampson may have been baptised at Gwennap in 1758!!! (I have not found a burial for Christian as yet so identification is still pending)
So, looking outside the box, and looking at the problem from different angles consider this .................
Assuming that I have identified Mary Martin (wife of Thomas Berryman) correctly then she was baptised at Towednack 14th March 1802 to parents James Martins and MARGARET BERRYMAN!!!
Margaret Berryman was baptised at Towednack 11th January 1775 ............ and guess who her parents were?
ANDREW BERRYMAN and SARAH QUICK
And Andrew Berryman and Sarah Quick just happen to be the parents of James Berryman who was baptised at Towednack 5th May 1771 thus making him a brother to Margaret (Berryman) Martin. So, could it be that rather than being the widowed father of Thomas he might actually have been the uncle of Mary Martin???
The Census gives his age as 83 and place of birth as Towednack so suggests he was born about 1767 or 1768. As you have pointed out James was died at Halsetown later in 1851 with his age recorded as 82 giving us 1768 or 1769 as a year of birth. And then we have to rely on the ages being accurate anyway. James was also brought back to Towednack for burial suggesting that was his home Parish.
So, depending on the accuracy of my earlier work on the Berryman/Berriman families the conclusion that James Berryman may have been the uncle of Mary Martin seems the most logical solution to the problem. The fact James and Christian Berriman named children Elizabeth and Mark certainly suggests I had the correct James for the marriage to Mary Martin in 1787.
CT
ANDREW BERRYMAN and SARAH QUICK 
