LOOKING FOR WILLIAM JONES born 1848 in Finsbury, Islington, London.

I know he was married in 1875 to Mary Cankett in Newport, Monmouthshire, Wales (I have certificate), and at that time his father DAVID was deceased. My problem is that I have yet to locate WILLIAM's birth certificate details. Many pounds(?) have been spent obtaining incorrect certificates!
I have to admit that I am not 100% sure that WILLIAM was born in London, but details from Ancestry.com suggest that he was so born!
There has been 'no joy' with census returns for 1841, 1851 and 1861, where I was hoping to locate the whole family before DAVID died.

Any help would be very much appreciated.