Thanks very much Gordon
I can think of 2 possibilities 1) that Joseph and Joshua are brothers and that James might be a cousin to David 2) that James and David are brothers, with Joshua as their father - not impossible given the vast families that people had then. Only thing is, David's first son was Joseph. He had 4 more sons none called Joshua. Given the naming formalities then, I would expect to have seen a Joshua - maybe he died inbetween censuses? Thanks again. Anne