I think I have the right family in 1841 and 1851,both times living in Lombard Street,Ynyscynhaiarn,with Henry's wife given as Margaret(and a daughter of that name as well)
1841 HO107/1391/10/41/30
1851 HO107/2512/549/51
In the grant of letters of administration of his will,William is described as"one of the children of.."
I have not yet been able to to find a full copy of the will(NLW finishes at 1858).
Roger