I have this to support the marriage
George Plant married Olive Ashmore
Name: PLANT, George
Registration district: Rochdale
Year of registration: 1929
Quarter of registration: Jan-Feb-Mar
Spouse's last name: Ashmore
Volume no: 8E
Page no: 91
Are you saying the GRO does not have this document?
There was a George Plant born in Rochdale, 8e 54, 1st quarter 1905, lived at 2 Bulwer St. Rochdale in 1911 with parents William Plant age 43 and Sarah Jane age 40, they had been married 21 years. Also there were Ernest 19, Fred 16, Beatrice 11 and Herbert 3.
It appears he died 2nd quarter 1958 in Rochdale, 10F 338, age 53.
I could find no divorce record.
Perhaps knowing more of your mothers siblings will give a better time frame. I did see a record of a George Plant being a prisioner of wat in Germany, but it not very conclusive