I don't doubt that she was from Cambridgeshire, as two censuses show that she was from Sawston. I have reservations that her name was Simmonds though, if you are basing your belief solely on the existence of the niece Maria Simmonds age 5 in 1851. In the absence of any supporting evidence, it's only a one in three chance that Elizabeth was also a Simmonds. Maria could equally have been the daughter of a married sister of either Jacob or Elizabeth, as opposed to the daughter of Elizabeth's brother. Do you have any other evidence to support your belief that Elizabeth was a Simmonds?
The Cambs FHS website at
http://www.cfhs.org.uk/Search.html has baptism and burial indexes 1801-37, but there's only one Symonds entry in Sawston - burial of Cowell of Lothbury, London age 27 in 1815, so Simmonds doesn't appear to be a Sawston name.
As she gave her ages as 57 and 67 I think you should be looking for an Elizabeth in Sawston 1793/4, but even if you find just one you're going to need the marriage entry to prove that you have the right one. If there's more than one Elizabeth baptised in Sawston in 1793/4 the marriage entry becomes even more essential.
David