Hi Nigel ...
Apologies for the delay in getting back to you, but I've now had a chance to go through my SYRETT files. Unfortunately, I don't seem to have anything that relates to William and Mary Anne.
I would just say that I understand that you've taken William's birth year of 1801 from his age (40) on the 1841 census. Of course, these ages are always rounded down to the nearest five years, so William could be any age from 40 up to 44 - and consequently born before 1800. There are several William SYRETTs recorded on the IGI born in Suffolk in the late 1790s which may be worth further investigation.
Sorry I couldn't be of more help.
Penny