The GRO index [free, just have to register] now includes mother's maiden names which makes life a lot easier ...
Searching from here https://www.gro.gov.uk/gro/content/certificates/indexes_search.asp reveals:
Name: Mother's Maiden Surname:
THOMAS, CATHERINE EMMA BOWEN
GRO Reference: 1885 D Quarter in NEATH Volume 11A Page 620
Note that census years of birth are backed out by subtracting the age from the census year so for those of us with a birthday in April or later (i.e. 75% of people) it will be off by a year.
Here's the corresponding link in the FreeBMD database (which doesn't have the mother's maiden name)
http://www.freebmd.org.uk/cgi/information.pl?cite=r6PlxsAVVZ6BvP8%2BfHdcIQ&scan=1
N.B. There is a second Catherine Emma Thomas registered in Q3 1886, also Neath, which could be confusing. She died in 1974 and from the death record I see her d.o.b. was 21st May 1886, mother's maiden name Morgan.
There certainly was a Rachel Bowen born in Cwm Avon around 1852. Here she is in the 1871 census with her parents John and Ann:
"England and Wales Census, 1871", database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:V5BJ-HLC : 24 July 2015), Rachel Bowen in entry for John Bowen, 1871
The transcription has many errors but as I subscribe to findmypast I can see the image and it is very hard to read but the birthplace is Cwm Avon. This Rachel's mother's maiden name was Mathews:
BOWEN, RACHEL MATHEWS
GRO Reference: 1851 D Quarter in OF NEATH Volume 26 Page 489
so her parents were John Bowen and Ann Mathews. However, I have not (yet) been able to verify whether she was Catherine's mother. I'll post more later.
Update:
I found Catherine and Phillip in the 1939 register living at 14 Park Row, Port Talbot, Glamorganshire. Their dates of birth are 27 August 1885 and 25 Feb 1883. I also found Catherine's death record from Q2 1974 which gives her d.o.b. as 17 August 1885. So one of them has an error - most likely the death record. This from www.findmypast.co.uk.
I think Heather made a slight error in her post. Thomas's wife (not necessarily Catherine's mother!) is called Elizabeth in 1901 (Philip's mother was called Martha).
"England and Wales Census, 1901," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:XSMS-4P6 : 29 August 2015), Thomas Thomas, Michaelstone Lower, Glamorganshire, Wales; from "1901 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing Margam subdistrict, PRO RG 13, The National Archives, Kew, Surrey
The plot thickens ... I think I've found Catherine in the 1891 census but there are two problems. First her age is given as 4 when in fact she would have been 5. Secondly her entry has been forced onto the first line of a new page and that has screwed up the data.
Her singleton entry is here:
"England and Wales Census, 1891," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:HMY3-Y2M : 14 October 2015), Catherine Thomas, Michaelstone Super Avon, Glamorganshire, Wales; from "1891 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing PRO RG 12, Glamorganshire county, subdistrict, The National Archives of the UK, Kew, Surrey
While the rest of the family are here:
"England and Wales Census, 1891," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:HMY9-G3Z : 15 October 2015), Thomas Thomas, Michaelstone Super Avon, Glamorganshire, Wales; from "1891 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing PRO RG 12, Glamorganshire county, subdistrict, The National Archives of the UK, Kew, Surrey
Catherine has unfortunately not been included in the family grouping however take my word she's just over the page. I've checked that Lily and Mary Ann both had a mother's maiden name of Bowen too and they all have a birthplace of Cwm Avon. The ages are Thomas (37), Rachel (39), Mary A (15), Lily (9)