I managed to get it working myself
I used this:
Code:
<if condition="in_array(THIS_SCRIPT, array(showthread)) AND $GLOBALS[threadinfo][replycount] <= 1">
<table style="margin-top: 8px;" width="100%" class="tborder">
<tr>
<td align="center">
No one has replied to this thread yet. Post a reply now.
</td>
</tr>
</table>
</if>
You can see that it doesn't show here:
http://www.rockonpublishing.co.uk/showthread.php?t=14
But it does here:
http://www.rockonpublishing.co.uk/showthread.php?t=15

Thanks for the help!