James - I have begun looking at your information and here follows my initial results.
Started with the 1851 Census looking for the Davey Family at Zennor and found only two. (One was a widowed mother with her daughter but I will look into that later.)
The other family was at Treen which is about half way between Porthmear and Boswednack:-
1851 Census - Zennor
Treen
John Davey, head, mar, 38, farmer of 18 acres land, St Just
Margaret Davey, wife, mar, 39, St Just
Jane Davey, daur, unm, 15, St Just
Elizabeth Ann Davey, daur, 12, St Buryan
Margaret Davey, daur, 10, St Buryan
John Davey, son, 7, St Just
Now - looking at the Richards family that I believe to be the one to which you refer.
Looks like we are linking in to some other discussions on this Forum.
If I am correct then Nancy Richards (nee Berryman) whom I believe to be the mother of your William, Thomas, John and David was the sister of Arthur and Thomas Berryman who are involved in another part of this Thread.
More from the 1851 Census for Zennor:-
Porthmear
William Richards, head, mar, 43, blacksmith, Zennor
Nanny Richards, wife, mar, 42, Zennor
William Richards, son, 14, Zennor
Thomas Richards, son, 11, Zennor
John Richards, son, 9, Zennor
Robert Richards, son, 8, Zennor
David Richards, son, 1, Zennor
It would appear probable that (IGI) William Richards was baptised at Zennor April 3rd, 1808 s/o John Richards.
I am yet to determine which John Richards this might be and also do not, as yet, know when and to whom he was married.
Before moving on to the 1861 Census I will take a look and see what I can find of the Davey family.
The following record from St Just 'may' be the marriage of John and Margaret from the 1851 Census:-
2 Mar 1835
John Davey of this parish
Margaret Leathan of this parish
Witnesses: John Boyns, John Daniel, John Tonkin Leatham
1861 Census Zennor
Boswednack
John Davey, head, widr, 47, farmer of 20 acres, St Just
Elizabeth A Davey, daur, unm, 23, assistant draper, St Buryan
Margretta Davey, daur, unm, 21, dairy maid, St Buryan
John Davey, son, unm, 16, farmer's son, St Just
Ann Mann, servt, unm, 25, house servant, Zennor
Elizabeth Mann, servt, unm, 23, house servant, Zennor
As John Davey is a widower in 1861 I would suggest the possibility that the following burial at Zennor is that of his wife Margaret:-
19 Feb 1853 Margaret Davey of Trewey aged 41
And now for the Richards in 1861.
1861 Census Zennor
Boswednack
William Richards, head, widr, 53, blacksmith, Zennor
John Richards, son, unm, 19, tin miner, Zennor
Robert Richards, son, unm, 18, tin miner, Zennor
David Richards, son, 11, scholar, Zennor
Eliza Thomas, servt, unm, 19, house servant, St Just
William Richards wife Nancy (or Nanny) (nee Berryman) was buried at Zennor October 5th 1852, age 43, of Porthmear.
Hope this lot helps and I will await your further postings before updating my database.
On the face of it I should think I have identified the correct people so let us work from there.