Hi Ed, is your site with this skin publicly visible? It's really not possible to debug CSS from screen shots.
If we can't get to your site, try FireBug (a FireFox addon). This is highly recommended for inspecting CSS elements to troubleshoot issues like this. You can right click on the place where you have extra space, and choose "inspect element." Then you can actually play interactively with the CSS settings to try and resolve it. Of course you'll still have to manually update the CSS on disk once you find the right spot to change.
Another tip is to make sure the following are set to false in your user.config. This way you will be able to find the exact file to change, and changes will be reflected immediately in the browser.
<add key="CombineCSS" value="false" />
<add key="MinifyCSS" value="false" />
<add key="CacheCssOnServer" value="false"/>
<add key="CacheCssInBrowser" value="false"/>
Once you are done troubleshooting, be sure to set all of these back to "true" for production.