I think your best route at this point is going to be via DNA or a "duplicate" birth cert ?

Child Surname Child Forename Mother's Former Name Year District Office Register Entry

SMITH Frederick Dennis 1925 Forest of Dean Monmouth, West Dean 8 93
SMITH Frederick Dennis 1925 Forest of Dean Westbury on Severn, East Dean 31 344

Year: 1927
Month: Apr
Day: 18
Grooms_Surname: NICHOLLS
Grooms_Forenames: Sidney John
Grooms_Age: 26
Groom_Condition: Bachelor
Grooms_Occupation: Collier
Grooms_Residence: The Scowles Coleford
Grooms_Fathers_Surname: NICHOLLS
Grooms_Fathers_Forenames: Albert John
Grooms_Fathers_Occupation: Labourer
Brides_Surname: SMITH
Brides_Forenames: Caroline Edith
Brides_Age: 20
Brides_Condition: Spinster
Brides_Occupation:
Brides_Residence: Whitecliff Coleford
Brides_Fathers_Surname: SMITH
Brides_Fathers_Forenames: Walter
Brides_Fathers_Occupation: Collier
Licence_or_Banns: Banns
Date_of_Banns:
Signature_or_Mark: Both sign
Witness_1: Walter Smith
Witness_2: M A Nicholl
Other_Witnesses: Elizabeth Smith
Officiating_Minister: Edward H Brice
Event: Marriage
Memoranda:
Notes:
Register_Reference: P93/1 IN 1/13
Page_Number: 92
Parish_Chapel: Coleford

1939 Scowles Rd. Coleford
Sidney J Nicholls 06 Apr 1901 Male Colliery Hewer Married 85 1
Caroline E Nicholls 17 Sep 1907 Female Incapacitated Married 85 2
Frederick O Nicholls 22 Apr 1925 Male General Labourer Single 85 3
Albert S I Nicholls 03 Sep 1927 Male At School Single 85 4
Charles W Nicholls 24 Dec 1929 Male At School Single 85 5
Robert H Nicholls 04 Feb 1932 Male At School Single 85 6