The expert on your query would be Peter Newcombe(son of Sheila Knill) who you will find responding to earlier messages.
His tree shows the John Knill son of John and Mary Roberts as born 1 Mar 1729 at Ilfracombe and marrying Grace How of and in Berrynarbour 6 Nov 1759.
I visited the churchyard there and their grave stones are on the right of the path to the church door but the dates are slightly different. They had a son John born Ilfracombe 10 Oct 1764 and George who is the subject of earlier messages onthis page.
I only found the grave of Isaak(Snr) at Ilfracombe but must admit the size of the graveyard deterred me! I join the Newcombe tree two generations before Thomas and Martha being descended from one Lewis Knill of Brauntonwho received £100 in his father's will , the other sons getting the farms.