Blamcca,
I am afraid I am somewhat confused here,
I note you mention William and Mary were born in South Petherwin, yet the St Gluvias couple according to census were William b Mylor about 1796 and Mary St East/St Erth about 1801. 1841 census children William 16, Mary 14, John 12, Elizabeth 10 and Jane 4.
Now there is a South Petherwin couple born about the same time as you suggest yet these are not the parents of the St Gluvias Children whose parents were William a Blacksmith and Mary.
This St Petherwin William was an ag lab, suggested birth Nth Petherwin Devon. Mary b South Petherwin Children in 1841 Jane 15, John 10, Maria 8 and James 5. (possibly William PEARSE and Mary ROOSE mar 29 Mar 1815 South Petherwin)
https://familysearch.org/pal:/MM9.3.1/TH-1-16101-61724-93?cc=1769414&wc=MMVH-VHX:n1229294109Certainly not the same couple. North and South Petherwin are near the border - nowhere near St Gluvias. Is the South Petherwin couple your family of interest or is it the St Gluvias couple?
Now you mention John - well both couples had sons John. John PEARSE 28 Mar 1830 South Petherwin to William and Mary.
Kris