Hi Malcom, yes my apologies your quite right that the specifics need to be worked out (including the assumption about the details of the crest). The quote from Johns book as follows on the origins of the family: "... My brother, who at one time made some enquiries respecting it, was of the opinion that it came from Norfolk ....... This belief was founded partly on the circumstance of an old family seal bearing the arms which belong to the Davys of Norfolk." - memoirs on the life of Sir Humphrey Davy - by sir John Davy - go to: http://archive.org/details/memoirslifesirh00davygoog
I agree certainly that more research would be needed to clarify the connection.
For my part - our Davy name originated nowhere near Norfolk or Cornwall. We are Welsh.
All the best.