Hi,
I think that I might have found the Catherine Mary WEBB that you are looking for although her father is Edward John WEBB but he uses John in the census records. She is born in 1891 so would be fibbing about her age on her marriage certificate and the passenger list but she changes her DOB between these two so she might just be aging slowly
Marriage: 1917 age: 24 born 1892/1893
List: 1923 age: 28 born 1894/95
so 6 years later she's only aged 4 years
The reason that I think this is a likely candidate for your Catherine Mary is that her sister Emily Elizabeth WEBB married a James ATKINS,
Emily Elizabeth WEBB Dec Q 1904 Canterbury 2a 1915
James Thomas ATKINS
James Thomas ATKINS WW1 pension records at Ancestry show his "Military History Sheet", this shows that his children were born in both Kent and Selby, Yorkshire. His address when he was transferred to the Army Reserve on 14 April 1919 was 40 Old Dover Rd Canterbury, (this has then been crossed out and a note that matches his later address has been added) when he was discharged on 31 March 1920 his address was 250 Barlby Rd, Selby, Yorks.
If the "younger brother in Selby in 1921 under the surname WEBB" found by Debra is correct Catherine might have been in Selby to have her child staying with or near her sister Emily.
The address on the emigration passenger list, 2 Roff Yard, James Street, Selby, is only 0.5 miles from 250 Barlby Road.
This Catherine also has a sister Alice that could be her marriage witness, and a brother called Alfred John which might have caused the marriage certificate name addition, if it was done later someone might have confused the name.
MarriageEdward John WEBB Jun Q 1882 Canterbury 2a 1238
Annie Eliza FASSOM
Children's birthsAmelia Prebble WEBB Dec Q 1882 Canterbury 2a 803
mmn: FASSOM
Emily Elizabeth WEBB Sep Q 1882 Bridge 2a 785
mmn: FASSOM
(I can't find this one in census or death records)
Emily Elizabeth WEBB Jun Q 1884 Bridge 2a 796
mmn: FASSOM
Alfred John WEBB Mar Q 1887 Elham 2a 1006
mmn: FASSON
Annie Louisa WEBB Jun Q 1889 Elham 2a 1003
mmn: FASSOM
Baptism
07 Apr 1889
St Peter, Folkestone, Kent
Annie Louisa WEBB
Father: Edward John
Mother: Annie
Catherine Mary WEBB Mar Q 1891 Elham 2a 1032
mmn: FASSON
1891 RG12/50 f.82 p.84 Folkestone, Kent (Elham R.D.)
28 Young's Rd
WEBB
John Head 36 General Labourer
Annie Wife 28 Canterbury Kent
Amelia Daur 9 Scholar " "
Emily Daur 7 " " "
John Son 4 " Folkestone Kent
Annie Daur 2 " "
Catherine Daur 3m " "
More children's birthsEdward Charles WEBB Mar Q 1894 Elham 2a 1034
mmn: FASSOM
Death
Edward Charles WEBB Jun Q 1894 Elham 2a 551
Age: 0
(GRO site says 6, but from previous experience seems unable to distinguish month and years, using years when it should be months)
Edith Ellen WEBB Sep Q 1896 Elham 2a 1004
mmn: FASSON
Alice ? I can't find a birth for her.
William Henry Sep Q 1900 Blean 2a 895
mmn: FASSOM
1901 RG13/796 f.13 p.17 Staplegate, Kent (Blean RD)
4 Staplegate Place
WEBB
Annie Head Mar 38 Charwoman
Emily Daur 19
Alfred Son 14
Kate Daur 10
Edith Daur 4
Alice Daur 3
Willie Son 9m all dittoed from top of page as Canterbury
More children's birthsCaroline Susannah WEBB Mar Q 1904 Bridge 2a 955
mmn: FASSOM
Baptism
07 Aug 1904
St Gregory, Canterbury, Kent
Caroline Susanna WEBB
Father: Edward John
Mother: Annie Eliza
Matilda Jane WEBB Mar Q 1908 Bridge 2a 994 mmn: FASSONS
We can't transcribe the 1911 census here due to copyright but you could look up the family in Littlebourne, Kent.
It says they have had 11 children 9 have survived which would be right if Emily Elizabeth was recorded twice or Alice isn't John and Annie's child.
I've just noticed that James Thomas ATKINS and Emily Elizabeth WEBB have a daughter ATKINS, Katherine M R ATKINS, she marries and is with her mother in 1939 as Kathleen M R (married surname), then her death is registered with yet another spelling, Katharine.
Sorry if this post is quite long but it's amazing how interesting a puzzle can be when you are looking out of the window at pouring rain when you should be out in the garden planting vegetables!
Regards,
Daisy