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The name seems to be primarily from North America

There is NO record of any sort on Ancestry for the name William Hunstable

No record, either, on 1911 census

no records at all showing for William Hunstable

I think the problem lies in the absence of a marriage record for Ethel's parents

this is on ancestry
Susan Joan Down
Birth About 1900
Parents & Siblings
Spouse & Children
William Frank Hunstable (1900-)
Living Hunstable
Timeline 1900
Birth
.

Ethels marriage in 1938 has her surname as Hunstable or Davies . what is the signficance of that ??

VERNEY, Harry Llyod Lloyd Head Married M 39 1872 Groom In Waiting To The King Esher Surrey VIEW
CANNON, Annie Servant Single F 27 1884 Housemaid Windlesham Surrey VIEW
CARTER, Daisy Servant Single F 17 1894 Kitchenmaid Botolph Claydon Bucks VIEW

Address:
1 Rutland Gardens County:
London

so they used mmn as part of Ethels names
Name: Ethel J D Hunstable
Mother's Maiden Surname: Down
Date of Registration: Oct Nov Dec 1920
Registration district: Newtown
Registration county: Wales
Volume Number: 11b
Page Number: 234

Not sure what you mean about finding your grandparenst. You seem to know a lot about them already.
You can find their marriage on FreeBMD and then work back from there. You will need to buy their marriage cert for ages and fathers names

My mother Ethel Joan Down Hunstable was born in 1920 (14 Nov) in Clochfaen Hall ,Llangurig,Montgomeryshire.Her birth certificate shows her father as William Hunstable and mother as Susan Down Hunstable. 
Clochfaen Hall was the home of Harry Lloyd Lloyd Verney and Lady Joan Elisabeth Mary Lloyd Verney (who my mother was named after) .William Hunstable and Susan Down Hunstable were Gardener And Cook to the Lloyd Verney family at Clochfaen Hall in 1920
In 1910 Harry Lloyd Verney was deputy Master of the household of Edward VII and then Groom in waiting to George V and in 1919 he also became the private secretary to Queen Mary .At that time Lady Joan Lloyd Verney was Lady in waiting to Queen Mary. When in London the Lloyd Verney family lived at 1 Rutland Gardens and at Buckingham palace.They also lived at 14 Hinde street,Manchester square W ,and at Carriden house,Bo'ness .
Can anyone helpme find my grandparents