I found a reference to Sparnick here:
http://books.google.com/books?id=-9UHAAAAQAAJ&pg=PT31&am...Note: This lists both Sparnick and Little Sparnick.
Ideally you would find this book at a library to see the map, BUT I haven’t been able to locate this (yet.)
Also, this is from some sort of fairy tale, but it sounds as if the location is "legit":
"While in Truro we better look out for the small folk on Sparnick Downs. Late one night about the year 1810 a certain journeyman taylor, named William Dunn, was returning to his home at Sparnick Downs, near Truro. Just as he was passing Kea churchyard he was startled..."
Also, in the various ship manifests for John and Mary Louisa, Truro is cited as either a destination or residence. It's likely that John was visting his family home near Truro....
I also found some info on the etymology of the name Chegwidden here:
http://books.google.com/books?id=4PO-ZFzSuvAC&pg=PA25&am...I thought it would be helpful to see these name variations.
Also, here's a couple of good Cornwall websites for you:
http://www.cornwall-opc.org I did not have any luck with finding John's baptism here, assuming he was born Feb. 1849 -- BUT I don't know if children were baptized soon after being born...? In some countries/areas, baptisms could be years later...?
Free Cornwall Census data:
http://www.freecen.org.uk/cgi/search.plThere are several John Chegwiddens (with surname variations) that could be yours IF you allow some leeway with the birth years.