I suspect that you are so immersed in this that I will not add anything!
The 1819 of William shows that he owed businesses at Burslem, Gresley and Belper.
Do you know when he was born?
Hugh was born 1772 wasn't he? And his grandfather was John Bourne from Abbey Farm near Milton.
I suppose that if the trees meet up then it would have to be around that point?
I agree that it looks as though William Jnr was the son of William Snr, and since the rest of the brothers appear to be in the Belper area then it looks as though William Jnr had moved to Burslem to look after his father's business. William Snr was in Belper by 1782 at least
http://www.belper-research.com/people/slater/thomas%20slater%20history.pdf and a bit before that. I suppose the question is, did he come from the Burslem area? He could have retained a business interest there, or inherited from someone after he moved to Belper? or do you think that the Belper Bournes had been there much earlier?