A copy of your fathers birth cert would give their names.
If you haven't got that, or cant get it from your father or any of his descendents, then you can send for a copy from
www.gro.gov.uk. First look it up on freebmd to get the reference number.
You coud try just looking for a Marshall/Williamson marriage on freeBMD, at the right time in the right area, but that would not be so reliable, especially if there was more than one!
Just seen your latest post - you have tried that one!