I saw your request for help on a Genes Reunited forum via a google search which provided more info.
In case you don't already have them the certificate references are below so you can get them from the GRO..
Your mum's birth:
Name: MOSS, Sally M
Registration district: Bournemouth
County: Dorset
Year of registration: 1940
Quarter of registration: Jan-Feb-Mar
Mother's maiden name: Wilkins
Volume no: 2B
Page no: 1823
Your mum's parent's marriage:
Groom: MOSS, Percy S
Bride: WILKINS, Sarah M
Registration district: Bournemouth
County: Dorset
Year of registration: 1937
Quarter of registration: Jul-Aug-Sep
Spouse's last name: Moss
Volume no: 2B
Page no: 2139
Your maternal grandfather's birth:
Name: MOSS, Percy S
Registration district: Christchurch
County: Hampshire
Year of registration: 1915
Quarter of registration: Oct-Nov-Dec
Mother's maiden name: Chapple
Volume no: 2B
Page no: 1049
There is a possible maternal grandmother's birth record in London.
If she was much older than your grandad and lied about her age at the marriage - which was very common back then...
Name: WILKINS, Sarah Maud
Registration district: St. Olave Bermondsey
County: London
Year of registration: 1903
Quarter of registration: Apr-May-Jun
Mother's maiden name: Not available before 1911 Q3
Volume no: 1D
Page no: 208
However, checking the 1911 census this girl is at home in 8, Silwood Street, Bermondsey with her father Christian Wilkins (so maybe not).