The Birth Certificate should give the name of the mother and can be ordered online at:
https://www.gro.gov.uk/gro/content/certificates/menu.asp

At the moment GRO is running a pilot supplying certificates in PDF format at a cost of £6.00. Details required are as follows:
Name: Mother's Maiden Surname:
ELLWAY, WILLIAM RILEY -
GRO Reference: 1881 S Quarter in MONMOUTH Volume 11A Page 21 Order