I think this is your grandparent's marriage in 1933:
Name: James R R Sanger
Event Type: Marriage Registration
Event Place: Lewisham, London, England
Registration District: Lewisham
County: London
Registration Year: 1933
Registration Quarter: Apr-May-Jun
Affiliate Line Number: 137
Volume: 1D
Spouse Name (available after 1911): Rolf
Possible Spouse:
Ellen A Rolf
Source: "England and Wales Marriage Registration Index, 1837-2005," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:QV8H-DGFS : 8 October 2014), James R R Sanger and null, 1933; from "England & Wales Marriages, 1837-2005," database, findmypast (http://www.findmypast.com : 2012); citing 1933, quarter 2, vol. 1D, p. 2051, Lewisham, London, England, General Register Office, Southport, England.
Birth registration for your grandfather in 1911:
SANGER, JAMES RICHARD ROBERT Mother's Maiden Surname: FINCH
GRO Reference: 1911 J Quarter in CAMBERWELL Volume 01D Page 858
Source: GRO Online Index https://www.gro.gov.uk/gro/content/certificates/indexes_search.asp
Birth registration for your grandfather's brother in 1915:
SANGER, FREDERICK CHARLES Mother's Maiden Surname: FINCH
GRO Reference: 1915 J Quarter in CAMBERWELL Volume 01D Page 1646
Source: GRO Online Index https://www.gro.gov.uk/gro/content/certificates/indexes_search.asp
Your great-grandparent's marriage in 1910:
Name: James Richard Sanger
Event Type: Marriage
Event Date: 25 Dec 1910
Event Place: Walworth, Surrey, England, United Kingdom
Age: 21
Marital Status: Single
Birth Year (Estimated): 1889
Father's Name: James Richard Sanger
Spouse's Name: Mabel Amelia Finch
Spouse's Gender: Female
Spouse's Age: 18
Spouse's Marital Status: Single
Spouse's Birth Year (Estimated): 1892
Spouse's Father's Name: Robert Finch
Source: "England, Surrey Parish Registers, 1536-1992," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:QGZY-QV1J : 16 March 2018), James Richard Sanger and Mabel Amelia Finch, 25 Dec 1910; citing Marriage, , London Metropolitan Archives, England; FHL microfilm 291,751.