Thanks for the suggestion @wceolin
I found the following in functions.php which seems to be the code I need to replace:
<div class="comment-meta">
<p>
<?php
/* translators: 1: comment author url, 2: comment author name, 3: comment permalink, 4: comment date/timestamp*/
printf( __( '<a href="%1$s" rel="nofollow">%2$s</a> said on <a href="%3$s"><span class="time-since">%4$s</span></a>', 'buddypress' ), get_comment_author_url(), get_comment_author(), get_comment_link(), get_comment_date() );
?>
</p>
</div>
I don’t know how to replace the code in functions.php – I’m happy to look up how to do that, but before I do I just wanted to check this is definitely the code I need to replace and I’m not barking up the wrong tree!