Welcome to Rootchat Sarah.
The original posted has not been on Rootchat since 2012.
So if they have the same email address may get a notification of your query.
You may already have some or all of this;
But posted as a Tree I looked at did not have Henry 1767's Parents.
I had a look at Sylvia on FreeREG;
Marriage 31 Jul 1859, Father STEPHEN
Her Baptism 1838 to Stephen/Sarah
Marriage if Stephen 1832 to Sarah LANGHAM
Stephens Baptism 10 Apr 1814, Foulsham to HENRY/MARY, Siblings John/Rachel/Simon/Sophia, earlier Children born Salle to them.
Marriage of Henry to Mary VOULT 1791 Salle
(FreeREG)
Henrys Baptism;
HARRY Sisson, 10 May 1767, St Peter & St Paul, Salle to HARRY/LYDIA (other Children to Henry/Lydia, 1st Elizabeth 1752)
(FreeREG)
Henry SISSON Married Lydia LOCKSMITH, 30 September 1751, Salle, Norfolk
(Familysearch.Org)
LYDIA Sisson appears to be Buried Fouksham, 30 April 1812 age 86 (1726)
Likely Baptism at Foxley 5 Dec 1725 to THOMAS/ELIZABETH Locksmith (they had a earlier Lydia who must have died)
(FreeREG)
Thomas married Elizabeth LEAMAN, 03 November 1717, St Thomas, Foxley
(FreeREG)
Trish

Sorry just realised you have the Sisens back to 1710 (Henry Buried 1792 age 81/82)