Hi BDV,
I have no knowledge of your line bar this thread and this site's PRs, so forgive me if I'm making a huge gaff. Have you used the rather nifty "Advanced Search" facility, it's very useful ?. You can enter the bare minimal of info, eg type "Jo" and amongst other names it will find all likely spellings of "Johnathun" etc. The list of findings can then be quickly sorted in various ways as you like. I used it to search all this site's Marriages between "John Bullock & Sarah", it only produced seven of which this is the only "possible" for parents of John born 4 May 1740 and to me seems a reasonable stab, what do you think ?.

Record ID 40782
Entry Number
Year 1735
Month Oct
Day 1
Grooms Surname BULLOCK
Grooms Forenames John
Grooms Age
Groom Condition
Grooms Occupation
Grooms Residence Minsterworth
Grooms Fathers Surname
Grooms Fathers Forenames
Grooms Fathers Occupation
Brides Surname WILLIAMS
Brides Forenames Sarah
Brides Age
Brides Condition
Brides Occupation
Brides Residence Minsterworth
Brides Fathers Surname
Brides Fathers Forenames
Brides Fathers Occupation
Licence or Banns licence
Date of Banns
Signature or Mark
Witness 1
Witness 2
Other Witnesses
Officiating Minister Jack[ma]n Morse rector
Event Marriage
Memoranda
Notes
Register Reference P184 IN 1/1
Page Number 37
Parish Chapel Huntley

I was wondering why they married at nearby Huntley and not Minsterworth ?. This interesting history of Minsterworth church perhaps answers it. http://www.wosb.org.uk/minsterworth_church.asp

As I suspected the current building is Victorian, "replacing the medieval(!) building whose fabric had badly deteriorated not least from the effects of flooding. It is recorded that in 1770 flood waters in the church resulted in the parish chest and all its contents being saturated..."
so maybe the Church wasn't being used regularly in 1735 either ?.

Again using the Advanced Search but for Baptisms, I find:

254280 1736 BULLOCK Sarah, John & Sarah, Minsterworth
254335 1740 BULLOCK John, John & Sarah, Minsterworth
254445 1752 BULLOCK Mary, John & Sarah, Minsterworth

Plus 12 more over the following century in nearby Westbury On Severn & Huntley who may be related.

From this site's PRs, I could not find an "obvious" Baptism for Sarah in Minsterworth, altho there are several "possibles" within the nearby parishes particularly on the Severn; unfortunately Sarah Bullock is a very popular name locally, as indeed is John Bullock which dates back before 1600 particularly in these Severnside parishes.

I cannot find a Burial for Sarah.
While searching I did find the following Marriage which is perhaps Sarah & John's aforementioned daughter Sarah, to a "local" man which I think should read Osbourn(e), assuming the witness Mary Bullock is her younger sister (see above).

Record_ID: 110437
Entry_Number: 202
Year: 1792
Month: Sep
Day: 30
Grooms_Surname: ORSBOURN
Grooms_Forenames: William
Grooms_Age:
Groom_Condition: Batchelor
Grooms_Occupation:
Grooms_Residence: Tainton Gloucestershire
Grooms_Fathers_Surname:
Grooms_Fathers_Forenames:
Grooms_Fathers_Occupation:
Brides_Surname: BULLOCK
Brides_Forenames: Sarah
Brides_Age:
Brides_Condition: Spinster
Brides_Occupation:
Brides_Residence: Parish of St Mary De Crypt
Brides_Fathers_Surname:
Brides_Fathers_Forenames:
Brides_Fathers_Occupation:
Licence_or_Banns: Licence
Date_of_Banns:
Signature_or_Mark: Both sign
Witness_1: Mary Bullock
Witness_2: William Webley
Other_Witnesses:
Officiating_Minister: Thomas Stock
Event: Marriage
Memoranda:
Notes:
Register_Reference: P154/11 IN 1/8
Page_Number: 52
Parish_Chapel: Gloucester St Mary de Crypt
Soundex_Groom: O621
Soundex_Bride: B420

For completeness John's Baptism & Burial Records from this site, as you know:

Record_ID: 254335
Entry_Number:
Year: 1740
Month: May
Day: 4
Parents_Surname: BULLOCK
Child_Forenames: John
Fathers_Forenames: John
Mothers_Forenames: Sarah
Mothers_Surname:
Residence:
Occupation:
Officiating_Minister: Edward Draper
Event: Baptism
Memoranda:
Notes:
Register_Reference: P218 IN 1/2
Page_Number: 4
Parish_Chapel: Minsterworth
Soundex: B420

Record_ID: 164013
Entry_Number:
Year: 1783
Month: Mar
Day: 2
Surname: BULLOCK
Forenames: John
Residence:
Age_at_death:
Officiating_Minister:
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P218 IN 1/2
Page_No: 26
Parish_Chapel: Minsterworth
Soundex: B420

I do hope this helps a little.