Suzanne,
Well Benjamin's father is named John; OK it'd be nice if it said John Oliver - but that's not to say that it isn't Harriet's father. Look at it this way, if it wasn't John then I'd say there was an issue.
Where it tells you Mary's father's name does it just say William, or does it say William Sumers or William Day.
The Willliam Day you mention, was he a witness ie. ' in the presence of..' if so that does not necessarily mean it's her father - it could be Benjamin's best man; and what was the other witness's name please.
Regards John