Thought I'd do a summing up, to see where we are.
On 25 May 1833 John Symons and Jane Trembath married at Gulval. Witnesses were Francis Symons and William Trembath.
John and Jane had 4 children baptised at Gulval, with residence stated as Trevarrack, and John's occupation stated as Miller. These were:
29 April 1834 Jane Symons
29 August 1835 Ann Symons (PB)
26 February 1836 John Symons
3 September 1838 William Symons (PB)
23 September 1838 William Symons (with note: PB 3.9.1838)
In 1841 we find the John (age 30) and Jane (age 35) at Trevarrack, Gulval with daughter Jane age 7, son John age 4 and son William age 2. But, also at the house are Thomas Symons, age 60, Miller, b. Cornwall, and William Major age 70, Ag Lab, born Cornwall.
On 3 May 1844 in the West Briton there is an article regarding a fire in Gulval at the house of John Symons, the cause of which was a spark falling onto the thatched roof. The roof fell in.
In 1851 we find the family living at Boskinnal, Ludgvan. John is now 45 years old, born at Madron, and Jane is 44, born at St Just. Also in the household are Jane age 17, John age 15 and William age 13, all born at Gulval.
Now I have a gap... I cannot find the family in 1861 or 1871. Any help to find them would be lovely
By 1881 John Symons in a widower and is a Boarder with the Donnithorne family at 45 New Street, Madron. He is age75.
I believe that Jane Symons (nee Trembath) died age 75 in the 4th Quarter of 1880 (5c, 185); and that John Symons died age 75 in the 4th Quarter of 1882 (both Penzance RD).
Back to John Symons' parentage. I am definitely leaning towards Thomas Symons and Ann Hockin being the parents of John.
1 Their John was born in the right time period;
2 The father was called Thomas and there is a Thomas with
John and Jane in 1841;
3 John and Jane called their second daughter Ann;
4 Thomas and Ann had a son called Francis (as Londoner
found
; a Francis Symons was a witness at the marriage of
John Symons and Jane Trembath.
Now, for Jane Trembath. Is there a possibility that she was christened Jenefer? I have seen instances before of Jenefer/Jane being interchangeable. There is a Jenefer Trembath baptised on 18 November 1804 at St Just, to parents William Trembath and Mary.
Thoughts/comments/corrections anyone?
(Oh, and just as an aside, thinking of your earlier Kessell enquiry, I found the following baptism at Gulval, which might be of interest:
21 September 1828, Ann Kestle Symons, dau of Samuel & Maria Symons, Chyandower, Wheelwright)
Maybe not, as John and Jane called one of their children Ann? Was their last child called William after Jane Trembath's father? And who was Francis Symons? Curiouser and curiouser 
