Do you know if John and Humphrey had any brothers. Did they have avy uncles who would have been Robert's brothers.
My great-grand father William Edward Davy, born in 1879, thought he was related to Sir Humphrey Davy and had some old family keepsakes that he thought came down from Humprey.
If he was related to John and Humphrey, his father Houton Davy who problably was born in the 1850-60, would probably have been a grandson or great-grandson of Robert or John or a male sibiling.
If would apprecaited any information that you might have.