P.S. It is possible to achive the output I want by hacking the common/widgets.php file, (not that I am suggesting this is good, as will be overwritten on upgrades!)
in widgets.php, changing the Topics widget query on lines 743 and 764 as below does what I want! Maybe not too hard for author to add customise this properly?
HACK:
// Order by most recent replies
case 'freshness' :
$topics_query = array(
'post_type' => array(bbp_get_topic_post_type(),bbp_get_reply_post_type() ), //HACKED!
// 'post_type' => bbp_get_topic_post_type(),