if this is the family in 1851, it's Betsy whose place of birth is Gwithian - other census entries also show this
1851 North of Trevassack, Phillack HO107/1916 folio 662 pg 47
Thomas Hosking head mar 31 labr in iron foundry
Betsy wife 31 b. Gwithian
Mary Ann dau 8
Agnes dau 7
Thomas son 5
Elizabeth Jane dau 2
Philippa dau 1
all born Phillack except Betsy
Thomas with family in 1841
Ventonleague, Phillack HO107/143/13 folio 38 pg 14
Thomas Hosking 50 ag lab
Mary 50
Thomas 20 labourer
James 18 labourer
Jane 15
Mary 12
Benjamin 8 ag lab
all born in county
Possible marriage 6 Apr 1810 in Phillack of Thomas Hosking & Mary Carthew
http://www.rootschat.com/links/01gdn/ If Elizabeth is the daughter of Thomas & Agnes, the marriage was Thomas Hosking & Agnes Carthew 21 Jan 1815 in Gwithian.
1841 census Trevarnon, Gwithian
Thomas Hosking 50 ag lab
Agnes 55
Phillip 20 Sailor
all born in county
Agnes bp 26 Sep 1785 Gwithian dau of John & Hester Carthew
Among her siblings Mary bp 18 Oct 1788 who went on to marry the other Thomas Hosking.
There are burials in 1790 in Gwithian for John Carthew Jnr in November & Snr in December. Cornwall Record Office has the will for John Carthew of Gwithian 1791 ref AP/C/2406 which if it is the will of the older John may mention provision for his grandchildren.