sortablegridfield/templates/Includes/GridFieldSortableRows.ss

<tr>
    <th class="extra sortablerowsheading" colspan="$Colspan">
        <div class="gridfield-sortablerows">
             <input type="checkbox" value="1"$Checked/> Allow drag and drop re-ordering
             $SortableToggle
             $PagenatorToggle
        </div>
    </th>
</tr>