Some of the census entries (18451/51) I have seen for the William down in Cornwall, who married Fanny (Frances) Luke, put his birth year closer to 1820-1. Just about old enough if he shaved another couple of years off his age, and worth just keeping at the back of your mind, for him to have been Joseph's father.....or brother!
Did William Snr show up anywhere in Joseph's life down in Cornwall - thinking here as a sponsor on children's christenings etc. or anything else.
Struggling to see anything as yet for Joseph back in Scotland with a birth year 1835 +/- 3 years.
Monica