Glanford Brigg looks to be the likely Registration District for Margaret's birth. Given that Margaret Robinson will turn up lots of entries try narrowing it down with the Registration District. Did Margaret have a middle name, do you have her mother's maiden name?
Quick search on GRO website included these entries:
Name: ROBINSON, MARGARET PRISCILLA
Mother's Maiden Surname: DRAYTON
GRO Reference: 1918 J Quarter in GLANFORD BRIGG
Volume 07A Page 1043
Name: Mother's Maiden Surname:
ROBINSON, MARGARET WHERRIT
GRO Reference: 1921 D Quarter in GLANFORD BRIGG Volume 07A Page 1188
ROBINSON, MARGARET PEARSON
GRO Reference: 1922 D Quarter in GLANFORD BRIGG Volume 07A Page 1160
ROBINSON, MARGARET ALICE PORTESS
GRO Reference: 1920 S Quarter in GLANFORD BRIGG Volume 07A Page 1362