Hi MPG,
glad to see you back onboard, hope all is well with you.
I must apologise for not posting my efforts, as poor as they are, when researching this post late yesterday. I thought of you M as it seems to me the witnesses are indeed key, sorry for not posting sooner.

For what they're worth here are all my findings on this query, was trying to find some common / useful ground but not a lot I'm afraid, the Whites seem to be an elusive family (altho I deliberately only searched STIBBS not Stubbs etc). Hopefully this info may help you or others, apologies for any duplication, this is what I found from the PRs and also FamilySearch etc.

From this site's PRs;

Record_ID: 124727
Entry_Number:
Year: 1809
Month: May
Day: 22
Grooms_Surname: STIBBS
Grooms_Forenames: James
Grooms_Age:
Groom_Condition:
Grooms_Occupation:
Grooms_Residence: of this Parish
Grooms_Fathers_Surname:
Grooms_Fathers_Forenames:
Grooms_Fathers_Occupation:
Brides_Surname: WHITE
Brides_Forenames: Mary
Brides_Age:
Brides_Condition:
Brides_Occupation:
Brides_Residence: of this Parish
Brides_Fathers_Surname:
Brides_Fathers_Forenames:
Brides_Fathers_Occupation:
Licence_or_Banns: Banns
Date_of_Banns:
Signature_or_Mark: Both mark
Witness_1: The mark of John Warlock
Witness_2: The mark of Dorcas Warlock
Other_Witnesses:
Officiating_Minister: W[illia]m Jones curate
Event: Marriage
Memoranda: Monday
Notes:
Register_Reference: P209 IN 1/8
Page_Number: 145
Parish_Chapel: Lydney
Soundex_Groom: S312
Soundex_Bride: W300

Record_ID: 337359
Entry_Number:
Year: 1813
Month: Jul
Day: 20
Parents_Surname: STIBBS
Child_Forenames: Sarah
Fathers_Forenames: James
Mothers_Forenames: Mary
Mothers_Surname:
Residence: of this parish
Occupation: labourer
Officiating_Minister:
Event: Baptism
Memoranda:
Notes:
Register_Reference:
Page_Number:
Parish_Chapel: Monmouth
Soundex: S312

Children;

Record_ID: 289845
Entry_Number: 137
Year: 1815
Month: Dec
Day: 24
Parents_Surname: STIBBS
Child_Forenames: Hester
Fathers_Forenames: James
Mothers_Forenames: Mary
Mothers_Surname:
Residence: Lidney
Occupation: Labourer
Officiating_Minister: Wm. Jones
Event: Baptism
Memoranda: D[aughte]r of
Notes:
Register_Reference: PFC 209 IN 1/3
Page_Number: 18
Parish_Chapel: Lydney
Soundex: S312

Record_ID: 125018
Entry_Number: 38
Year: 1840
Month: Aug
Day: 22
Grooms_Surname: SHEPHERD
Grooms_Forenames: George
Grooms_Age: Of full age
Groom_Condition: Bachelor
Grooms_Occupation: Labourer
Grooms_Residence: Lydney
Grooms_Fathers_Surname: SHEPHERD
Grooms_Fathers_Forenames: John
Grooms_Fathers_Occupation: Labourer
Brides_Surname: STIBBS
Brides_Forenames: Sarah
Brides_Age: Of full age
Brides_Condition: Spinster
Brides_Occupation: Servant
Brides_Residence: Lydney
Brides_Fathers_Surname: STUBBS
Brides_Fathers_Forenames: James
Brides_Fathers_Occupation: Navigator
Licence_or_Banns: [not stated]
Date_of_Banns: [not stated]
Signature_or_Mark: Both mark
Witness_1: The mark of Alexander Shepherd
Witness_2: The mark of Esther Shepherd
Other_Witnesses:
Officiating_Minister: Charles Taylor
Event: Marriage
Memoranda:
Notes:
Register_Reference: P209 IN 1/15
Page_Number: 19
Parish_Chapel: Lydney
Soundex_Groom: S163
Soundex_Bride: S312

Struggling to find them in the Census', but do have;

Name Sarah Shepheard
Event Type Census
Event Date 1841
Event Place Lydney, Gloucestershire, England
Residence Note Village Newern
Gender Female
Age 25-29
Age (Original) 25
Birth Year (Estimated) 1812-1816
Birthplace Gloucestershire
Registration District Chepstow
Parish Lydney
County Gloucestershire
Page Number 5
Registration Number HO107
Piece/Folio 350/18
Affiliate Record Type Household

From Familysearch which is my only references at this time, and in this instance one of those "odd" individual returns they occasionally throw-up, so unable to see the rest of this Census page, I wonder if other family members are on it ??

But evetually found this,

Name George Shephard
Event Type Census
Event Date 1841
Event Place Lydney, Gloucestershire, England
Residence Note Village Of Newern
Gender Male
Age 25-29
Age (Original) 25
Birth Year (Estimated) 1812-1816
Birthplace Gloucestershire
Registration District Chepstow
Parish Lydney
County Gloucestershire
Page Number 4
Registration Number HO107
Piece/Folio 350/18
Affiliate Record Type Household

Some of the names below fit the witnesses at Sarah Stibbs' marriage. Is Ester WHITE possibly Ester Sheppard's mother ?. Suspect "Elic Sheppard" (male) = Alec aka Alexander Sheppard, witness ?

1841 Althorp House, Lydney, Gloucestershire, England
Household Role Sex Age Birthplace
Ester White F 70-74
Elic Sheppard M 40-44
Ester Sheppard F 20-24 Gloucestershire
Thomas Sheppard M 7 Gloucestershire
George Sheppard M 5 Gloucestershire
William Sheppard M 4 Gloucestershire
Charles Sheppard M 2 Gloucestershire
John Sheppard M 0 Gloucestershire
Betsy Reece F 10 Gloucestershire
James Prosser M 10 Gloucestershire

http://www.forest-of-dean.net/fodmembers/index.php?mode=thread&id=17732

Record_ID: 201655
Entry_Number: 1058
Year: 1873
Month: Apr
Day: 28
Surname: SHEPHERD
Forenames: George
Residence: Tutshill Lane
Age_at_death: 57
Officiating_Minister: John J.Trollope
Event: Burial
Cause_of_death:
Memoranda: [note in Name field] By Warrant from Coroner
Notes:
Register_Reference: P209 IN 1/12
Page_No: 133
Parish_Chapel: Lydney
Soundex: S163

So born abt 1816, which fits nicely.

Record_ID: 202963
Entry_Number: 256
Year: 1895
Month: Nov
Day: 11
Surname: SHEPHERD
Forenames: Sarah
Residence: Lydney
Age_at_death: 80
Officiating_Minister: J.C.E.Besant
Event: Burial
Cause_of_death:
Memoranda:
Notes: [Besant underlined]
Register_Reference: Lydney P209 IN 1/14
Page_No: 32
Parish_Chapel: Lydney
Soundex: S163

So born abt 1815, which fits nicely.