Further to my post about email address’s needing spam protection Neil pointed me in the right direction. Spam_protect=1 needs to be set (it already was) but to avoid displaying email address’s this needs to be combined with show_email=”0″
Therefore my Comment Listing Template now contains the line
<span class=”comments-post”>Posted by <$MTCommentAuthorLink show_email=”0″ spam_protect=”1″$> at <$MTCommentDate$></span>