Anyone any idea who this gal is? 
Births Sep 1939 Saunders    Rachael    Penberth    Eton    3a   2307
There is a marriage in 1958 Newton Abbott for a namesake of hers 
Children (all girls) seems to be S K, E C , A & C M (registered 1963-is this you Cassie?)
In which case, we need to take a step closer to present-day, before going back in time.
Can you give us full parent details & witness names from the 1958 marriage certificate
What may be Rachael's  siblings  registered as MMN Penberthy & Pemberthy (1927-1941) 
Last birth (a female- name starts with B) in Edmonton (No Gillian or other suggested names though)
Marriages Sep 1926  
PENBERTHY    Dorothy V    Saunders    Taunton    5c   632     (b 1908?)
Saunders    Edwin G    Penberthy    Taunton    5c   632  (b 1904?)
PENBERTHY, DOROTHY  VIOLET     WAY  1908  J Quarter in TAUNTON  Volume 05C  Page 276
Deaths 1946 Saunders    Dorothy V    39    Plymouth    7a   635
SAUNDERS, EDWIN  GEORGE     MILLS     1904  D Quarter in TAUNTON  Volume 05C  Page 265
Deaths Mar 1977   SAUNDERS    Edwin George    10OC1904    Bristol    22   576
It MAY be that Gillian etc are simply a foster family that Rachael was raised with .
Did you ever meet your grandfather, or remember his death date.
If you were born when I think you were, and I have the right guy, you would have been in secondary school at the time.