Calvin,
I've only just come across your query.
There is a marriage, in the Stonham Aspall parish registers, between William Sillett and Mary Clark, on 6th September 1767. I think that these are your William's parents. I have four children for them:-
Elizabeth Sillett, baptised 27 March 1768
Hannah Sillett, baptised 2 July 1775
William Sillett, bapt. 21 December 1777, bur. 13 March 1779 and
William Sillett, bapt 6 July 1783.
William Sillett and Jemima Sheldrake were married on 31 December 1781 in Mickfield, and had 9 children that I know of. I am pretty confident that this is a different William.
Mike.