For anyone that has the same issue, I managed to find the CSS and resolve my positioning issue.
The following code was used to remove the top and bottom padding, which then left a gap as described.
ul.sf-menu li a {
padding: 0 25px 0 0;
}
The following additional code allows you to adjust the position of the floating menu, the figure provided being the height of the nav row itself. I took a screen capture and counted the row height in pixel
.sf-menu li:hover ul {
top: 18px;
}
Hope this helps future readers
Andy