I’m using a Sass-based theme, so access to the theme files from wp-admin won’t be useful. I’ve tried adding margin-top:20px in the inspector. It just bumps the entire grid down. I’ve tried this on your demo site as well, same thing happens, entire grid moves down, space between the items stays the same.
When you say “The top are set by the masonry js” do you mean in the wmljs.js file?