Hi Dave,
The IGI is showing William's bapt. here (a shrunken link);
http://www.rootschat.com/links/0t6n/  But its only a submitted entry.
I haven't any 
baptism  transcripts for Sherston Magna but in the 
marriage records there is a marriage for;
John Sherwood to  Mary Dickenson 14-5-1776  that is a possible as I couldn't see any others
before William's birth in 1781.
In case you missed it, William's surname is listed as Shearwood when he married  Ann Stickler 29-3-1802 in Sherston.
Other Shearwoods who married around the 1800s are James 1809 (Ann Hawkins) & 1815 as a widower Mary Rice, John  1814 (Mary Neal), Robert 1814 (Anne Evans),  
Sherwood; Thomas 1804 (Sarah Power)
All in Sherston Magna.
Hope these help.
Alan