You should get a copy death certificate from didcot register office,
Register Office: 197 Broadway Tel: 0845 129 5900
and it should give you William's age at death his occupation and address and the informant, probably your partners mum, later registrations included the deceased date and place of birth too .
when you have the certificate you could go to the birth registrations quite easily, then you will have his fathers name to confirm which is the right marriage, though you should be able to do that if you know your mum's maiden name, which is on your partners birth certificate.