Can you confirm please.....your mother's birth was recorded as Phyllis May Duncan PEREIRA, in Kolkata, India, 1912.
What names do you see for her parents?
Are there siblings....who might also drop the PEREIRA name in later life?
Your mother arrived in England 1948 as Phyllis May DUNCAN ....an unmarried woman?
And married in England 1948+ as Phyllis May DUNCAN?