On the 1947 electoral register entry there's a Margaret Ambrosina Hickson at the same address as Horace and Maria.
Margaret appears to be the same Margaret/Margherita that married Reginald Frank Hickson in 1939 and then re-married in 1946 to a Walter G Manners, Reginald having died in WWII in Singapore. Margaret appears to have been born in 1915, so if she's a daughter of Horace and Maria you're likely looking for a marriage before then.
GRO references:
1939 Q2 Marriage: Reginald F Hickson & Margherita A Millard; Paddington; vol 1a; page 38
1946 Q2 Marriage: Walter G Manners & Margaret A Hickson; Westminster; vol 1a; page 1205
1997 Q4 Death: Walter George Manners; age 86 (born c1911); St Albans (5351a); A47C; 170
2011 Q1 Death: Margaret Ambrosina Manners; age 96 (born c1915); Hertfordshire (538-1r); 504396471
(I'm not certain about Walter's death but it looks kinda right. The will could provide confirmation. And either marriage certificate could provide confirmation that Horace is Margaret's father.)
Probate:
Walter George Manners
died 27 Oct 1997
https://probatesearch.service.gov.uk/Wills?Surname=manners&SurnameGrants=manners&YearOfDeath=1997&YearOfDeathGrants=1997&IsGrantSearch=True&IsCalendarSearch=False&page=2#willsProbate:
Margaret Ambrosina Manners
died 22 Jan 2011
https://probatesearch.service.gov.uk/Wills?Surname=manners&SurnameGrants=manners&YearOfDeath=2011&YearOfDeathGrants=2011&IsGrantSearch=True&IsCalendarSearch=False#willsElectoral registers:
1947 - 26 Upper Richmond Road, Putney; Margaret A Manners, Walter G Manners
1949 - 26 Upper Richmond Road, Putney; Margaret A Manners, Walter G Manners
1950 - 26 Upper Richmond Road, Putney; Margaret A Manners, Walter G Manners
1965 - 17 Chester Avenue, Richmond Upon Thames; Margaret Manners, Walter G Manners
Prisoner of war record:
name: Reginald Frank Hickson
birth: 25 Jan 1913
father: Albert Lewis Hickson
mother: Sophia Louise Jeanette Hickson
place of origin: 24 Knox Street, Marylebone, London
destination of report: Mrs M A Hickson, 17 Leinster Mews, Lancaster Gate, Bayswater
CWGC
name: Reginald Frank Hickson
age: 31
died 24 Jun 1944
https://www.cwgc.org/find-records/find-war-dead/casualty-details/2133629/REGINALD%20FRANK%20HICKSON/EDIT: Added 1965 electoral register (possible) record for Margaret and Walter