One young William Fortt can be seen in the 1861 census at 5 Milsom Street, with his sisters Emma and Emily and cousin Sophia Fisk (head of household absent). He is a Confectioner's Assistant, aged 29, born in Bath and unmarried.
In the same census William K Fortt is at 33 Claremont Buildings, with his wife Emma and several children (one born in USA). He is a carpenter, 29 and born in Surrey.
That is enough to show that Emma's husband is certainly not the son of William and Mary and very likely is the son of John and Maria, (baptised at Southwark, Surrey in Feb 1832).
The other William seems to be living with his brother George in Islington in the 1881 and 1891 censuses, still unmarried. It looks like he never called himself William King Fortt (although there is a squiggle on the 1881 census which Ancestry have transcribed as a C, making him William C Fortt.)
(Oops, that's yet another William, perhaps son of Joseph and Amelia)
(5 Milsom Street is now part of a Waterstone's bookshop but in my youth it was Fortt's Restaurant. So it looks like it stayed in the family until the 1980s.)
David
