Hi, I had a look at this at Essex RO. First I had a look, as you have probably done already, at the marriage in 1818 in Braintree, St Michael
ref: D/P 264/1/35 fiche 2 of 4 page 51
George BROWN otp bach m by banns Sarah BENSON otp sp with consent of parents 3 Dec 1818. Both signed register
Witnesses were Can..h BROWN, Ann FAVER? Mary? B....., John COLLIS* and George FAVER?
*He seemed to be regular witness - perhaps parish clerk
I had a look at Chelmsford baptisms, as you requested, and saw no BENSON baptisms at all in the circa 1799. I did the same for Braintree baptisms with same result.
So I went back to Chelmsford baptisms D/P 94/1/4 (fiche no of 5 not noted)
and on page 199 was this baptism
1799 Feb 3 Sarah d of Mary WOLFE illegitimate by James PURKIS
If you feed into IGI Mary Wolfe, spouse name BENSON married 1800 + or - 20 years anywhere in England you get
Mary WOLFE m William BENSON on 4 June 1815 at Great Stambridge.
Unfortunately, this info has been put into IGI by a member of LDS after 1991
without quoting a reliable source so you need to look at Great Stambridge register before you can be reasonably certain that James PURKIS was a ratfink

and William BENSON was not