Buying their marriage certificate will give you their father's names and occupations and this should enable you to take your search back another generation. Here is the GRO reference:

Name: William Campion
Spouse Surname: Lees Or Holden
Date of Registration: Oct-Nov-Dec 1915
Registration District:Burton upon Trent
Volume Number: 6b
Page Number:932

Martha is indexed as both Lees and Holden, so there is a good chance she had been married before or that she was for some other reason using a surname different from that she was born with. The certificate should clarify this.

Make sure you buy it direct from the GRO (www.gro.gov.uk). Third party suppliers will charge considerably more than the statutory £9.25 fee and take considerably longer to supply it.