Darn it - for the benefit of anyone who has the quick reply mod installed, the installation should look like this...
<br /><br /><br /><br />### In showflat.php find this:<br /><br /><br /> $Parentlink = "";<br /> if ($ParentPost) {<br /> $postrow[$i]['Subject'] = "<a href=\"{$config['phpurl']}/showthreaded.php?Cat=$Cat&Board=$Board&Number=$ParentPost&page=$page&view=$view&sb=$sb&o=$o&vc=1\" target=\"_new\" title=\"{$ubbt_lang['RESPONSE_TO']}$ParentPost\" alt=\"{$ubbt_lang['RESPONSE_TO']}$ParentPost\">$Subject</a>";<br /> }<br /><br />else <br />{ <br />$postrow[$i]['Subject'] = "<span id=\"subject$i\">$Subject</span>";<br /> }<br /><br /><br />### and change it to this:<br /><br /><br /> $postrow[$i]['Subject'] = $Subject;<br /> $postrow[$i]['replyto_link'] = "<a href=\"{$config['phpurl']}/showthreaded.php?Cat=$Cat&Board=$Board&Number=$ParentPost&page=$page&view=$view&sb=$sb&o=$o&vc=1\" target=\"_new\" title=\"{$ubbt_lang['RESPONSE_TO']}$ParentPost\" alt=\"{$ubbt_lang['RESPONSE_TO']}$ParentPost\">";<br />$postrow[$i]['Subject'] = "<span id=\"subject$i\">$Subject</span>";<br /><br />
This ensures that both the 'Reply to Who' and the 'Quick Reply' mods both work

Ian