Hi Janet and a warm welcome to the forum !
Have you tried searching this site's Parish Records database ?
Struggling to find either marriage within this site's PRs, but using the Advanced Search for Baptisms can find the following seven Baptisms for John & Hannah which appear relevant to your search. I've also given a few other PRs for each person where they were easily found, sadly my time is limited today. Of course it's quite possible the men especially might have moved/married out of Westbury, maybe Gloucester etc which was easily reachable up the river if not further north.

Year Surname Child Forenames Parent's Forenames Parish Chapel
1763 BELCHER John.John & Hannah. Westbury on Severn
1766 BELCHER Joseph. John & Hannah. Westbury on Severn
1771 BELCHER Benjamin.John & Hannah. Westbury on Severn
1773 BELCHER Richard.John & Hannah. Westbury on Severn
1775 BELCHER Elizabeth.John & Hannah. Westbury on Severn
1777 BELCHER Mary.John & Hannah.Westbury on Severn
1781 BELCHER Sarah.John &Hannah.Westbury on Severn

The full PRs may be found within the database.
http://forest-of-dean.net/joomla/index.php/advanced-search

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Joseph's Marriage & Burial, perhaps ?
I suspect more relevant PRs may be found within the database as well as any I have found.

Record_ID: 206979
Entry_Number: 127
Year: 1793
Month: Jun
Day: 13
Grooms_Surname: BELCHER
Grooms_Forenames: Joseph
Grooms_Age:
Groom_Condition: [not stated]
Grooms_Occupation:
Grooms_Residence: otp [Westbury]
Grooms_Fathers_Surname:
Grooms_Fathers_Forenames:
Grooms_Fathers_Occupation:
Brides_Surname: HORWOOD
Brides_Forenames: Mary
Brides_Age:
Brides_Condition: [not stated]
Brides_Occupation:
Brides_Residence: otp [Westbury]
Brides_Fathers_Surname:
Brides_Fathers_Forenames:
Brides_Fathers_Occupation:
Licence_or_Banns: Banns
Date_of_Banns:
Signature_or_Mark: Both Mark
Witness_1: Mark of John Potter
Witness_2: Tobias Cowles
Other_Witnesses:
Officiating_Minister: William Boughton Minister
Event: Marriage
Memoranda:
Notes:
Register_Reference: P 354 IN 1/5
Page_Number: 32
Parish_Chapel: Westbury on Severn
Soundex_Groom: B426
Soundex_Bride: H630

Record_ID: 384410
Entry_Number: 459
Year: 1828
Month: Mar
Day: 10
Surname: BELCHER
Forenames: Joseph
Residence: Westbury
Age_at_death: 62
Officiating_Minister: William Crawley Curate
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P354 IN 1/8
Page_No: 58
Parish_Chapel: Westbury on Severn
Soundex: B426

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Benjamin b1771;

Record_ID: 384391
Entry_Number: 440
Year: 1827
Month: Oct
Day: 9
Surname: BELCHER
Forenames: Benjamin
Residence: Westbury
Age_at_death: 58
Officiating_Minister: William Crawley Curate
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P354 IN 1/8
Page_No: 55
Parish_Chapel: Westbury on Severn
Soundex: B426

(UPDATE, See my next post !)

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Richard b1773;

Record_ID: 385970
Entry_Number: 418
Year: 1857
Month: Dec
Day: 7
Surname: BELCHER
Forenames: Richard
Residence: Westbury
Age_at_death: 84
Officiating_Minister: CW Payne Crawford Off[iciatin]g Minister
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P 354 IN 1/16
Page_No: 53
Parish_Chapel: Westbury on Severn
Soundex: B426

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Elizabeth b1775;

A possible Marriage, altho' bear in mind Westbury is just one port on the River Severn, which in those days was a busy "highway" to nearby villages such as Broadoak, Newnham, Minsterworth and so on. Also Arlingham just across the River may be worth your attention, the site of a busy ferry to Newnham. Here you will find several Belcher PRs in this site's database, as well as those in Westbury. Until quite recent times it was entirely routine for many people to cross the river daily by this ferry, including children for school and suchlike.

Record_ID: 207012
Entry_Number: 160
Year: 1796
Month: Apr
Day: 17
Grooms_Surname: RIDER
Grooms_Forenames: John
Grooms_Age:
Groom_Condition: [not stated]
Grooms_Occupation:
Grooms_Residence: otp [Westbury]
Grooms_Fathers_Surname:
Grooms_Fathers_Forenames:
Grooms_Fathers_Occupation:
Brides_Surname: BELCHER
Brides_Forenames: Elizabeth
Brides_Age:
Brides_Condition: Spinster
Brides_Occupation:
Brides_Residence: otp [Westbury]
Brides_Fathers_Surname:
Brides_Fathers_Forenames:
Brides_Fathers_Occupation:
Licence_or_Banns: Banns
Date_of_Banns:
Signature_or_Mark: Both Mark
Witness_1: Tobias Cowles
Witness_2: Sarah Rider
Other_Witnesses:
Officiating_Minister: Henry Southouse Curate
Event: Marriage
Memoranda:
Notes:
Register_Reference: P 354 IN 1/5
Page_Number: 40
Parish_Chapel: Westbury on Severn
Soundex_Groom: R360
Soundex_Bride: B426

Possibly ?

Record_ID: 384553
Entry_Number: 601
Year: 1832
Month: Jun
Day: 10
Surname: RYDER
Forenames: Elizabeth
Residence: Westbury
Age_at_death: 57
Officiating_Minister: William Crawley Curate
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P354 IN 1/8
Page_No: 76
Parish_Chapel: Westbury on Severn
Soundex: R360

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Mary b1777;

this looks possible, altho perhaps a few years late for this Mary ??

Record_ID: 207224
Entry_Number: 27
Year: 1815
Month: Nov
Day: 13
Grooms_Surname: BENNETT
Grooms_Forenames: James
Grooms_Age: [Not stated]
Groom_Condition: Batchelor
Grooms_Occupation: [Not stated]
Grooms_Residence: Of this Parish
Grooms_Fathers_Surname: [Not stated]
Grooms_Fathers_Forenames: [Not stated]
Grooms_Fathers_Occupation: [Not stated]
Brides_Surname: BELCHER
Brides_Forenames: Mary
Brides_Age: [Not stated]
Brides_Condition: Spinster
Brides_Occupation: [Not stated]
Brides_Residence: Of this Parish
Brides_Fathers_Surname: [Not stated]
Brides_Fathers_Forenames: [Not stated]
Brides_Fathers_Occupation: [Not stated]
Licence_or_Banns: Licence
Date_of_Banns: [Not stated]
Signature_or_Mark: She signs He marks
Witness_1: Ann Young
Witness_2: John Belcher
Other_Witnesses: Anth[ony] Bennett
Officiating_Minister: Rich[ard] Wetherell Vicar
Event: Marriage
Memoranda:
Notes:
Register_Reference: P354 IN 1/7
Page_Number: 9
Parish_Chapel: Westbury on Severn
Soundex_Groom: B530
Soundex_Bride: B426

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For Sarah b 1781,

continued on next post.

The BELCHER family feature in my family tree. This is my best stab at their family history:

Generation No. 1

1. JOHN BELSHER/BELCHER/BELTSHAR baptised about 1691. Had issue:
1.1. JOHN BELSHER/BELCHER/BELTSHAR baptised 8th December, 1713, Arlingham.

Generation No. 2

2. JOHN BELSHER/BELCHER/BELTSHAR baptised 8th December, 1713, Arlingham. Had issue:
2.1. JOHN BELSHER/BELCHER/BELTSHAR born about 1730.

Generation No. 3

3. JOHN BELSHER/BELCHER/BELTSHAR married, as JOHN BELTSHER, on 25th June 1758, Arlingham HANNAH SIER, clearly a shotgun marriage since his son WILLIAM BELCHER was baptised on 13th August 1758, Arlingham. Had issue:
3.1. WILLIAM BELCHER baptised 13th August 1758, Arlingham; mentioned in the Will of his father JOHN BELSHER GDR 1809/222;
3.2. JOHN BELCHER baptised 27th December 1763, Westbury on Severn; mentioned in the Will of his father JOHN BELSHER GDR 1809/222 married MARY U/K.
3.3. JOSEPH BELCHER baptised 1st January 1766, Westbury on Severn; mentioned in the Will of his father JOHN BELSHER GDR 1809/222;
3.4. HANNAH BELCHER baptised 18th June 1769, Westbury on Severn;
3.5. BENJAMIN BELCHER baptised 12th May 1771, Westbury on Severn;
3.6. RICHARD BELCHER baptised 28th March 1773, Westbury on Severn.
4. JOHN BELCHER died intestate as stated in GDR 1809/222:
“27th day of December 1809 on which day appeared personally John Belcher and alleged that John Belcher late of the Parish of Westbury on Severn in the County of Gloucester Farmer deceased lately died intestate and that he the said John Belcher was and is the natural and lawful son and also next of kin of the said deceased whereof he prayed that Administration of all and singular the Goods and Chattells Rights and Credits of the said deceased might be granted and committed to him upon giving such good and sufficient security as in his behalf and so forth. Let Administration be granted as above prayed Charles Palmer sworn £100”.
Administration was granted to “Mr John Belcher of Westbury on Severn in the County of Gloucester Mason, Joseph Belcher of the same place Labourer and William Belcher of the same place Labourer”.

Generation No. 4

5. JOHN BELCHER (baptised 27th December 1763 in Westbury On Severn; Will Westbury-on-Severn GDR 1836/ 96 stonemason written on 7th June 1833) married on 6th January 1790, Westbury on Severn MARY MARTIN (mentioned in Will of her husband GDR 1836/96; mentioned in Will of her son DANIEL BELCHER GDR 1845/91; baptised 23rd June 1771 Westbury, parents HENRY MARTIN and MARTHA FLUCK). Had issue:
5.1. MARY BELCHER baptised 27th June 1790, Westbury on Severn;
5.2. JOHN BELCHER baptised 5th January 1792, Westbury on Severn; mentioned in Will of father GDR 1836/ 96;
5.3. BENJAMIN BELCHER baptised 12th May 1797, Westbury on Severn;
5.4. SUSAN BELCHER baptised 27th June 1802, Westbury on Severn;
5.5. DANIEL BELCHER baptised 24th January 1808, Westbury on Severn; Will GDR 1845/91 in which no children or wife is mentioned; mentioned in Will of his father GDR 1836/ 96.
6. The Will of JOHN BELCHER (GDR 1836/96) reads as follows:
“In the name of God Amen. I John Belcher of the Parish of Westbury on Severn in the County of Gloucester being in Good Health and of good and disposing mind and memory thanks be to God given do make and ordaine this my last will and testament in manner and form following that is to say I give unto my son Daniel Belcher my House Orchard and Gardens and all out Buildings and Conveniences whatsoever thereto belonging for the term of my lease together with that bit of land at the bottom thereof. I give unto Mary my wife all my goods and Chattels and everything I should die possessed of for her life after her death I give unto my son Daniel Belcher everything that remains except one Barrell I give my son John Belcher and one Hogshead I give unto my daughter Mary Bennett. My son Daniel Belcher shall also pay my debts and both our funeral expenses and I appoint my son Daniel Belcher Executor of this my last will and testament hereby revoking all others by me heretofore made witness whereof I have to this my last will and testament set my hand and seal this 7th day of June in the year of our Lord one thousand eight hundred and twenty three John Belcher signed sealed published and declared by the said Testator to be his Last Will and Testament in the presence of us who in his presence and in his presence subscribe our names as witnesses hereto. John Twitch Joseph Sparrow John Ravenhill.
The 11th June 1836
On this day appeared personally Daniel Belcher of Westbury on Severn in the County of Gloucester Mason and alleged upon oath that the foregoing paper writing contains the Last Will and Testament of John Belcher lately deceased who died on the 24th day of December leaving Goods Chattels wholly within the Diocese of Gloucester under the value of £20 that he is the sole Executor therein mentioned that he will well and faithfully perform the same and render an Inventory and account wherefore he prayed probate. Probate granted.”

Generation No. 5

7. JOHN BELCHER (baptised 5th January 1792, Westbury on Severn; buried 28th July 1871, Westbury on Severn, age 79. He married ELIZABETH JANE DAVIS on 19th July 1817, Chipping Sodbury, por: groom Chipping Sodbury, por: bride Dodington witnesses ANN SEDGE, WILLIAM HOWELL. ELIZABETH was buried on 10th April 1843, age 47, Westbury on Severn. Had issue (amongst others):
7.1. ANN BELCHER baptised 10th August 1823, Chipping Sodbury, Gloucestershire; a Farm Labourer, age 15, living in Newland, Gloucestershire at the time of the 1841 census, a Farm Labourer in the household of WILLIAM PROCTER. Bream is only 2 and a half miles from the residence of her future husband CHARLES JAMES in Oldcroft; baptised about 1824, Chipping Sodbury, Gloucestershire according to the 1851 census.
8. By the time of the 1851 census JOHN BELCHER, mason, 58, is a widower and lodging with MARY JONES in Northwood Tything in Westbury on Severn.
9. By the time of the 1861 census JOHN BELCHER is a stonemason, age 67, from Westbury on Severn, and is a Crime Prisoner in Gloucester prison.
10. By the time of the 1871 census he is a Pauper in the workhouse.

Generation No. 6

11. CHARLES JAMES married ANN BELCHER on 26th December 1842, Lydney. Had issue:
11.1. HARRIETT JAMES SNR. Had issue:
11.1.1. HARRIETT JAMES JNR m SAMUEL MORGAN.

I have the following on Hannah Sier;

1. JOSEPH SIER (born before 1711; buried on 15th December 1764, Westbury on Severn) married (before 1731) ELIZABETH U/K (buried as a widow on 26th March 1785, Westbury on Severn). Had issue:
1.1. SARAH SIER baptised 10th October 1730, Westbury on Severn;
1.2. ELIZABETH SIER baptised 5th March 1732, Westbury on Severn, Gloucestershire);
1.3. ELIZABETH SIER baptised 3rd May 1733, Westbury on Severn;
1.4. JOSEPH SIER baptised 3rd August 1735, Westbury on Severn;
1.5. HANNAH SIER baptised 27th November 1737, Westbury on Severn;
1.6. ABIGAIL SIER baptised 2nd February 1745, Westbury on Severn;
1.7. BENJAMIN SIER baptised 15th March 1747, Westbury on Severn;
1.8. RICHARD SIER baptised 1st December 1751, Westbury on Severn.