Could this Baptism be another clue?
William Henry 29 Sep 1869
Parents Surname Yemm
Father George
Mother Ann
Forest of Dean Bible Christian Circuit (Baptised at) Bible Christian Chapel Bream

Name: George Yemm
Year of Registration: 1838
Quarter of Registration: Jul-Aug-Sep
District: Westbury On Severn
County: Gloucestershire
Volume: 11
Page: 457

This YEMM maybe the YEMM family in Abenhall 1841 rather than Blakeney.
1841 Silver St
Catharina Yamam abt 1771 Abinghall, Gloucestershire
Thomas Yamam abt 1811 Gloucestershire, England Abinghall, Gloucestershire
Mary Yamam abt 1821 Gloucestershire, England Abinghall, Gloucestershire
Enock Yamam abt 1826 Gloucestershire, England Abinghall, Gloucestershire
George Yamam abt 1837 Gloucestershire, England Abinghall, Gloucestershire

I'll look a little deeper as time allows.

1871 Bream Woodside
George Yemm abt 1838 Blakeney, Gloucestershire, England Head Newland, Gloucestershire
Ann M Yemm abt 1841 Bream, Gloucestershire, England Wife Newland, Gloucestershire
Alfred Yemmabt 1862 Yockley, Gloucestershire, England Son Newland, Gloucestershire
Metha M Yemm abt 1849 Bream, Gloucestershire, England Son Newland, Gloucestershire
Ruth Yemm abt 1867 Bream, Gloucestershire, England Daughter Newland, Gloucestershire
William H Yemm abt 1870 Bream, Gloucestershire, England Son Newland, Gloucestershire

Name: Alfred Yemm
Year of Registration: 1862
Quarter of Registration: Jul-Aug-Sep
District: Monmouth (1837-1939)
County: Gloucestershire, Herefordshire, Monmouthshire
Volume: 11a
Page: _4

Name: Ruth Yemm
Year of Registration: 1867
Quarter of Registration: Apr-May-Jun
District: Monmouth (1837-1939)
County: Gloucestershire, Herefordshire, Monmouthshire
Volume: 11a
Page: 30

Name: William Henry Yemm
Year of Registration: 1869
Quarter of Registration: Jul-Aug-Sep
District: Monmouth (1837-1939)
County: Gloucestershire, Herefordshire, Monmouthshire
Volume: 11a
Page: 26

1881
Ann Maria Yemm abt 1841 Abersychan, Monmouth, Wales Head West Dean, Gloucestershire
Ruth Yemm abt 1869 Bream Woodside, Gloucestershire, England Daughter West Dean, Gloucestershire
William H. Yemm abt 1871 Bream Woodside, Gloucestershire, England Son West Dean, Gloucestershire

then you get to contend with !

Name: Meshach Imms
Year of Registration: 1858
Quarter of Registration: Oct-Nov-Dec
District: Monmouth (1837-1939)
County: Gloucestershire, Herefordshire, Monmouthshire
Volume: 11a
Page: 17

1861
George Imm abt 1838 Blakeney, Gloucestershire, England Head Newland, Gloucestershire
Ann Imm abt 1840 Abersychan, Monmouthshire, Wales Wife Newland, Gloucestershire
Mesech Imm abt 1859 West Dean, Gloucestershire, England Son Newland, Gloucestershire

JENKINS Ann Maria Dec 1857 Westbury Vol 6a Page 360

If all else fails you can always invest in the marriage certificate.

Can anybody help me identify the correct Surname?

Ann Maria was my gggrandfather Simeon Jenkins sister.